Introduction
The Shia shahadah (declaration of faith) states:
"There is no god but Alla'h, Muhammad is the Messenger of Alla'h, Ali' is the Friend of Alla'h. The Successor of the Messenger of Alla'h And his first Caliph."
If you are already familiar with standard Sunni beliefs, you will immediately notice the addition to the shahadah regarding Imam Ali (ra), cousin of the Prophet (pbuh), husband of his daughter Fatima, father of Hassan and Hussein and the second person ever to embrace Islam. The term Shia or Shi'ite derives from a shortening of Shiat Ali or partisans of Ali.

History
Ali is the central figure at the origin of the Shia / Sunni split which occurred in the decades immediately following the death of the Prophet in 632. Sunnis regard Ali as the fourth and last of the "rightly guided caliphs" (successors to Mohammed (pbuh) as leader of the Muslims) following on from Abu Bakr 632-634, Umar 634-644 and Uthman 644-656. Shias feel that Ali should have been the first caliph and that the caliphate should pass down only to direct descendants of Mohammed (pbuh) via Ali and Fatima, They often refer to themselves as ahl al bayt or "people of the house" [of the prophet].
When Uthman was murdered while at prayer, Ali finally succeeded to the caliphate. Ali was, however, opposed by Aisha, wife of the Prophet (pbuh) and daughter of Abu Bakr, who accused him of being lax in bringing Uthman's killers to justice. After Ali's army defeated Aisha's forces at the Battle of the Camel in 656, she apologized to Ali and was allowed to return to her home in Madinah where she withdrew from public life.
However, Ali was not able to overcome the forces of Mu'awiya Ummayad, Uthman's cousin and governor of Damascus, who also refused to recognize him until Uthman's killers had been apprehended. At the Battle of Suffin Mu'awiya's soldiers stuck verses of the Quran onto the ends of their spears with the result that Ali's pious supporters refused to fight them. Ali was forced to seek a compromise with Mu'awiya, but this so shocked some of his die-hard supporters who regarded it as a betrayal that he was struck down by one of his own men in 661.
Mu'awiya declared himself caliph. Ali's elder son Hassan accepted a pension in return for not pursuing his claim to the caliphate. He died within a year, allegedly poisoned. Ali's younger son Hussein agreed to put his claim to the caliphate on hold until Mu'awiya's death. However, when Mu'awiya finally died in 680, his son Yazid usurped the caliphate. Hussein led an army against Yazid but, hopelessly outnumbered, he and his men were slaughtered at the Battle of Karbala (in modern day Iraq). Hussein's infant son, Ali, survived so the line continued. Yazid formed the hereditary Ummayad dynasty. The division between the Shia and what came to be known as the Sunni was set.
An opportunity for Muslim unity arose in the 750's CE. In 750 except for a few who managed to flee to Spain, almost the entire Ummayad aristocracy was wiped out following the Battle of Zab in Egypt in a revolt led by Abu Al Abbass al-Saffah and aided by considerable Shia support. It was envisaged that the Shia spiritual leader Jafar As-Siddiq, great-grandson of Hussein be installed as Caliph.
But when Abbass died in 754, this arrangement had not yet been finalised and Abbas' son Al Mansur murdered Jafar, seized the caliphate for himself and founded the Baghdad-based Abbassid dynasty which prevailed until the sack of Baghdad by the Mongols in 1258.
Theological Differences and Attempts at promoting Unity
The line of Mohammed (pbuh) through Ali and Hussein became extinct in 873CE when the last Shia Imam, Muhammad al-Mahdi, who had no brothers disappeared within days of inheriting the title at the age of four. The Shias refused, however, to accept that he had died, preferring to believe that he was merely "hidden" and would return. When after several centuries this failed to happen, spiritual power passed to the ulema, a council of twelve scholars who elected a supreme Imam.
The best known modern example of the Shia supreme Imam is the late Ayyatollah Khomeni, whose portrait hangs in many Shia homes. The Shia Imam has come to be imbued with Pope-like infallibility and the Shia religious hierarchy is not dissimilar in structure and religious power to that of the Catholic Church within Christianity. Sunni Islam, in contrast, more closely resembles the myriad independent churches of American Protestantism. Sunnis do not have a formal clergy, just scholars and jurists, who may offer non-binding opinions.
Shias believe that their supreme Imam is a fully spiritual guide, inheriting some of Muhammad's inspiration ("light") . Their imams are believed to be inerrant interpreters of law and tradition. Shia theology is distinguished by its glorification of Ali. In Shia Islam there is a strong theme of martyrdom and suffering, focusing on deaths of Ali and, particularly, Hussein plus other important figures in the Shia succession. Shi`ism attracted other dissenting groups, especially representatives of older non-Arab (Mawali) civilizations (Persian, Indian, etc.) that felt they had not been treated fairly by the Arab Muslims.
Sunnis and Shias agree on the core fundamentals of Islam - the Five Pillars - and recognize each others as Muslims. In 1959 Sheikh Mahmood Shaltoot, Head of the School of Theology at Al Azhar university in Cairo, the most august seat of learning of Sunni Islam and the oldest university in the world, issued a fatwa (ruling) recognizing the legitimacy of the Jafari School of Law to which most Shias belong. As a point of interest, the Jafari School is named after its founder Imam Jafaf Sidiq who was a direct descendent through two different lines of the Sunni Caliph Abu Bakr. And Al Azhar University, though now Sunni, was actually founded by the Shia Fatimid dynasty in 969CE.
However, there remain significant differences between the two forms of Islam and these are what tend to be emphasized. Many Sunni's would contend that Shias seem to take the fundamentals of Islam very much for granted, shunting them into the background and dwelling on the martyrdoms of Ali and Hussein. This is best illustrated at Ashura when each evening over a period of ten days the Shias commemorate the Battle of Karbala, with a wailing Imam whipping the congregation up into a frenzy of tears and chest beating.
It is alleged that instead of missionary work to non-Muslims, the Shia harbor a deep-seated disdain towards Sunni Islam and prefer to devote their attention to winning over other Muslims to their group. There is ongoing violent strife between Sunnis and Shias in Pakistan. On the other hand, in recent years there has been signification co-operation between the two groups in the Lebanon. And some of the most dynamic developments in Islam today are taking place in Shia-dominated Iran.

Practical Differences
On a practicaldaily level, Shias have a different call to prayer, they perform wudu and salat differently including placing the forehead onto a piece of hardened clay from Karbala, not directly onto the prayer mat when prostrating. They also tend to combine prayers, sometimes worshipping three times per day instead of five. The Shias also have some different ahadith and prefer those narrated by Ali and Fatima to those related by other companions of the Prophet (pbuh).
Because of her opposition to Ali, those narrated by Aisha count among the least favored. Shia Islam also permits muttah - fixed-term temporary marriage - which is now banned by the Sunnis. Muttah was originally permitted at the time of the Prophet (pbuh) and is now being promoted in Iran by an unlikely alliance of conservative clerics and feminists, the latter group seeking to downplay the obsession with female virginity which is prevalent in both forms of Islam, pointing out that only one of the Prophet's thirteen wives was a virgin when he married them.

ShiasToday
Iran is overwhelmingly Shia - 89%.Shias also form a majority of the population in Yemen and Azerbaijan, Bahrain and 60% of the population of Iraq. There are also sizeable Shia communities along the east coast of Saudi Arabia and in the Lebanon. The well known guerilla organization Hizbollah, which forced the Israelis out of southern Lebanon in 2000, is Shia. Worldwide, Shias constitute ten to fifteen percent of the overall Muslim population.
Within Shia Islam there are different sects. Most Shias are "Twelvers", i.e. they recognize the 12 Imams. There are also Sevener and Fiver Shias who don't recognize the later Imams.
Please find below a commentary from a Shia encyclopaedia concerning Sheikh Shaltoot's fatwa plus the English translation of the fatwa itself. Both were originally posted on the One Ummah site where the original Arabic version of the fatwa is also available.
At the very bottom of this page, you will find what I understand to be a complete statement of Shia beliefs.
©2001 Islam For Today dot com

Al-Azhar Verdict on the Shia
What follows is the Fatwa (religious verdict/ruling) of one of the Sunni world's most revered scholars, Sheikh Mahmood Shaltoot with regard to the Shia. Shaikh Shaltoot was the head of the renowned al-Azhar Theological school in Egypt, one of the main centers of Sunni scholarship in the world. It should be of interest to know that a few decades ago, a group of Sunni and Shia scholars formed a center at al-Azhar by the name of "Dar al-Taqreeb al-Madhahib al-Islamiyyah" which translates into "Center for bringing together the various Islamic schools of thought". The aim of the effort, as the name of the center indicates, was to bridge the gap between the various schools of thought, and bring about a mutual respect, understanding and appreciation of each school's contributions to the development of Islamic Jurisprudence, among the scholars of the different schools, so that they may in turn guide their followers toward the ultimate goal of unity, and of clinging to one rope, as the well-known Quranic verse, "Hold fast to the Rope of Allah and do not diverge" clearly demands of Muslims.
This massive effort finally bore its major fruit when Sheikh Shaltoot made the declaration whose translation is appended below. It should be made unequivocally clear as well, that al-Azhar's official position, vis a vis the propriety of following any of the Madhaahib (schools of law), including the Shi'ite Imami school, has remained unchanged since Shaikh Shaltoot's declaration.
For the readership's reference the phrase "al-Shia al-Imamiyyah al-Ithna 'Ashariyyah" means the Twelver Imami Shi'ite School of thought which comprises the overwhelming majority of Shi'ites today. The phrase "Twelver Shi'ites" is used interchangeably with "Ja'fari Shi'ites" and "Imami Shi'ites" in various literature. They are merely different names for the same school of thought.
"al-Shia al-Zaidiyyah" are a minority among the Shi'ites, concentrated mainly in Yemen located in the Eastern part of Arabian peninsula. For a more detailed description of the Zaidis vs. the Twelver Shi'ites, please refer to the book, "Shi'ite Islam" written by the great Shi'ite scholar, Allamah Tabataba'i, and translated by Seyyed Hossein Nasr, and published by the State University of New York Press (SUNY).
And as for ShaikhShaltoot's declaration ...

Fatwa (ruling) of Shaikh Mahmood Shaltoot
Head Office of al-Azhar University:
IN THE NAME OF ALLAH, THE BENEFICENT, THE MERCIFUL Text of the Verdict (Fatwa) Issued by His Excellency Shaikh al-Akbar Mahmood Shaltoot, Head of the al-Azhar University, on Permissibility of Following "al-Shia al-Imamiyyah" School of Thought
His Excellency was asked:
Some believe that, for a Muslim to have religiously correct worship and dealing, it is necessary to follow one of the four known schools of thought, whereas, "al-Shia al-Imamiyyah" school of thought is not one of them nor "al-Shia al-Zaidiyyah." Do your Excellency agree with this opinion, and prohibit following "al-Shia al-Imamiyyah al-Ithna Ashariyyah" school of thought, for example?
His Excellency replied:
1) Islam does not require a Muslim to follow a particular Madh'hab (school of thought). Rather, we say: every Muslim has the right to follow one of the schools of thought which has been correctly narrated and its verdicts have been compiled in its books. And, everyone who is following such Madhahib [schools of thought] can transfer to another school, and there shall be no crime on him for doing so.
2) The Ja'fari school of thought, which is also known as "al-Shia al- Imamiyyah al-Ithna Ashariyyah" (i.e., The Twelver Imami Shi'ites) is a school of thought that is religiously correct to follow in worship as are other Sunni schools of thought. Muslims must know this, and ought to refrain from unjust prejudice to any particular school of thought, since the religion of Allah and His Divine Law (Shari'ah) was never restricted to a particular school of thought. Their jurists (Mujtahidoon) are accepted by Almighty Allah, and it is permissible to the "non-Mujtahid" to follow them and to accord with their teaching whether in worship (Ibadaat) or transactions (Mu'amilaat).
Signed, MahmoodShaltoot.
The above Fatwa was announced on July 6, 1959 from the Head of al-Azhar University, and was subsequently published in many publications in the Middle East which include, but are not limited to:
al-Sha'ab newspaper (Egypt), issue of July 7, 1959. al-Kifah newspaper (Lebanon), issue of July 8, 1959.
The above segment can also be found in the book "Inquiries about Islam", by Muhammad Jawad Chirri, Director of the Islamic Center of America, 1986 Detroit, Michigan.

FUNDAMENTALS OF FAITH OF THE SHI'I' IMAMI' ITHNA ASHARI'
Compiled by Ilya's Isla'm

THE SHAHADAH: THE DECLARATION OF FAITH
La ila'hail Alla'h, Muhammadan Rasu'l Alla'h, Ali'yun Wali'-Alla'h, Wasi'yu Rasulillah, wa Khali'fa tuhu bila fasl. There is no god but Alla'h, Muhammad is the Messenger of Alla'h, 'Ali' is the Friend of Alla'h. The Successor of the Messenger of Alla'h And his first Caliph.

USUL AL-DI'N: THE FUNDAMENTALS OF ISLAM
1) Tawhi'd(The Oneness of Alla'h)
2) 'Adl (Divine Justice)
3) Nubuwwah (The Prophethood)
4) Ima'mah (The Imamate)
5) Qiya'mah (The Day of Judgement)

FURU AL-DIN: THE MAIN BRANCHES OF ISLAM
1) Salat (Prayer)
2) Sawm (Fasting)
3) Zaka't (Poor-due of 2.5%)
4) Hajj (Pilgrimage to Makkah)
5) Khums (The Charity of 20%)
6) Jihad (To Struggle in the Path of Alla'h)
7) Amr bil ma'ruf (To Promote the Good)
8) Nahy 'an al-munkar (To Forbid the Wrong)
9) Tawalla (Loving the Prophet's Family)
10) Tabarra (Shunning the Enemies of the Prophet's Family)

THE PROPHETS OF ALLA'H (mentioned in the Holy Qur'a'n)
1) Adam
2)Idri's (Idrees)
3) Nu'h (Noah)
4) Hu'd
5) Sa'lih
6) Ibra'hi'm (Abraham)
7) Isma''i'l (Ishmael)
8) Isha'q (Isaac)
9) Lu't (Lot)
10) Ya'qu'b (Jacob)
11) Yu'suf (Joseph)
12) Shu'aib
13)Ayu'b (Job)
14) Mu'sa (Moses)
15) Ha'ru'n (Aaron)
16) Dhu l-kifl (Ezzekiel)
17) Dawu'd (David)
18) Sulaima'n
19) Ilya's (Elijah)
20) al-Yasa' (Elisha)
21) Yu'nus (Jonas)
22) Zakari'ya (Zakariyah)
23) Yahya' (John the Baptist)
24) 'I'sa (Jesus)
25) Muhammad
In a famous hadith (prophetic tradition), the number of prophets given was 124 000. May the blessings of Alla'h be upon them all. Prophethood ended with Muhammad (peace be upon him and his progeny). Then, Alla'h deputed Imams to guide us.

THE LAW-BRINGING PROPHETS
1) Nu'h
2) Ibra'hi'm
3) Mu'sa
4) 'I'sa
5) Muhammad

THE BOOKS OF ALLA'H
1) Sahi'fa(scroll revealed to Nuh)
2) Sahi'fa (scroll revealed to Ibra'hi'm)
3) Taurat (the book revealed to Mu'sa)
4) Zabu'r (the psalms revealed to Dawu'd)
5) Inji'l (the gospel revealed to 'Isa)
6) Qur'a'n (the Koran revealed to Muhammad)

THE PANJATAN: THE FIVE HOLY ONES
Muhammad,Fa'tima al-Zahra, 'Ali', Hasan, Husayn

THE FOURTEEN MASUMIN: THE RIGHTLY-GUIDED
Muhammad,Fa'tima al-Zahra and the Twelve Imams

THE TWELVE IMAMS
Name Title Year of birth and death
1) Imam 'Ali' ibn Abu Talib al-Murtadha (The Satisfied One) 600–661
2) Imam Hasan ibn Ali al-Mujtabah (The Chosen One) 625–669
3) Imam Husayn ibn Ali Sayyid al-Shuhudah (The Lord of the Martyrs) 626–680
4) Imam 'Ali' ibn Husayn Zayn al-A'bidi'n (The Jewel of the Believers) 658–713
5) Imam Muhammad al-Ba'qir (The Spreader of Knowledge) 676–743
6) Imam Ja'far al-Sa'diq (The Truthful One) 703–765
7) Imam Mu'sa al-Kazim (The Patient One) 745–799
8) Imam 'Ali' al-Ridha' (The Accepted One) 765–818
9) Imam Muhammad al-Taqi' (The Pious One) 810–835
10) Imam 'Ali' al-Naqi' (The Pure One) 827–868
11) Imam Hasan al-Askari' (The One with an Army) 846–874
12) Imam Muhammad al-Mahdi' (The Rightly-Guided One) 868—
The Twelth Imam is still alive. He is in a state of occultation. He will reappear at a moment determined by Alla'h. He is the Awaited One who will spread justice throughout the world.

THEPROFESSION OF FAITH OF THE TWELVER SHI'I
I bear witnessthat there is no god but Alla'h and that Muhammad, peace be upon him, is His servant and Messenger, and that 'Ali', the Commander of the Faithful, and the Chief of the Deputies of Alla'h, is the Imam whose obedience has been made incumbent by Alla'h on all people; and that Hasan and Husayn, 'Ali' ibn al-Husayn, Muhammad ibn 'Ali', Ja'far ibn Muhammad, Musa ibn Ja'far, 'Ali' ibn Musa, Muhammad ibn 'Ali', 'Ali' ibn Muhammad, Hasan ibn 'Ali', and the Living One, the Mahdi' (the blessings of Alla'h be upon them all), all the Imams of the believers and the Proofs of Alla'h for the whole of creation are my Imams, the rightly-guiding and the pious. I bear witness that: Alla'h is my God, Muhammad is my Prophet, Islam is my religion, the Qur'a'n is my scripture, the Ka'aba is my qibla, 'Ali' ibn Abu' Ta'lib is my Imam, Hasan ibn 'Ali' is my Imam, Husayn, the Martyr of Karbala, son of 'Ali', is my Imam, 'Ali' Zayn al-'A'badi'n is my Imam, Muhammad al-Ba'qir is my Imam, Ja'far al-Sa'diq is my Imam, Musa al-Ka'dhim is my Imam, 'Ali' al-Ridha' is my Imam, Muhammad al-Taqi' is my Imam, 'Ali' al-Naqi' is my Imam, Hasan al-Askari' is my Imam, and al-Huja al-Muntazar is my Imam. They, upon whom be peace, are my Ima'ms, Masters and Intercessors before Alla'h. I love them, all of them, and shun their enemies in this life and the next.
I bear witness that: Alla'h, the Almighty, the Exalted, is the best Lord; that Muhammad, the blessings of Alla'h be upon him and his Family, is the best Prophet; and that the Commander of the Faithful, 'Ali' ibn Abu' Ta'lib, and his offspring, are the best Imams; and that the message Muhammad brought from Alla'h is true, death is true, the questioning in the grave by Munkar and Naki'r is true, the Resurrection of the dead is true, the appearance before Alla'h is true, the Bridge (al-sira't) is true, the Divine Scales are true, the dissemination of the book of one's deeds at Doomsday is true, paradise is true, and hell is true; and that there is no doubt about the coming of the inevitable Hour of Reckoning; and that the rising of the dead from their graves is true.

THE POSITIVE ATTRIBUTES OF ALLA'H
1) Qadi'm:Alla'h is eternal. He has neither a beginning nor an end.
2) Qadir: Alla'h is omnipotent. He has power over all things.
3) 'Alim: Alla'h is omniscient. He is all-knowing.
4) Hai: Alla'h is living. He is alive and will remain alive forever
5) Muri'd: Alla'h has his own discretion is all affairs. He does not do anything out of compulsion.
6) Mudrik: Alla'h is all-perceiving. He is all-hearing, all-seeing, and is omnipresent. Alla'h sees and hears everything though he has neither eyes nor ears.
7) Mutakalim: Alla'h is the Lord of the Worlds. He can create speech in anything: the burning bush for Musa and the curtain of light for Muhammad.
8) Sadiq: Alla'h is truthful. His words and promises are true.

THE NEGATIVE ATTRIBUTES OF ALLA'H
1) Shari'k:Alla'h has no partners.
2) Murakab: Alla'h is neither made, nor composed, of any material.
3) Maka'n: Alla'h is not confined to any place and has no body.
4) Hulu'l: Alla'h does not incarnate into anything or anybody.
5) Mahale hawadith: Alla'h is not subject to changes. Alla'h cannot change.
6) Mari': Alla'h is not visible. He has not been seen, is not seen, and will never be seen, because he has no form or body.
7) Ihtiyaj: Alla'h is not dependant. Alla'h is not deficient, so he does not have any needs.
8) Sifate zayed: Alla'h does not have added qualifications. The attributes of Alla'h are not separate from His being.