INTRODUCTION
There is no difference of opinion amongst Muslims that the ‘Religion’ of God is Islam; that the only way to know Islam is through the Book of God and the Sunnah of The Holy Prophet (pbuh&hf); that the Book of God is what is known as The Qur'an, without any addition or subtraction. And what difference is there, is in the interpretation of some of the verses of Qur'an; and in believing or not believing some of the Sunnah as genuine; or in its interpretation.
This difference of approach has led towards the difference in some basic principles and some laws of Shariah. As the basic principles of Islam are well known, I do not think it necessary to enumerate all the beliefs. It will be sufficient if some of the important differences are described here to give the readers a fairly comprehensive idea of the main characteristics which distinguish Shia from the Sunnis.
All the Muslims agree:
1) Allah is One(swt)
2) Mohamrnad is His last Prophet(pbuh&hf)
3) That one day Allah (swt) will resurrect all the human beings and all will be answerable to their beliefs and actions.
All of them agree that anyone not believing in any of the above Three basic Principles is not a Muslim.
Also, they agree that anybody denying the famous tenets of Islam, like Prayers, Fasting, Hajj, Zakat, etc., or believing that the famous sins, like drinking wine, adultery, stealing, gambling, lie, murder, etc., are not sins, is not a Muslim, though he might have been believing in Allah (swt) and His Prophet Mohammed (pbuh&hf), because to deny such things is tantamount to deny the Prophethood of Mohammed and his Shari'ah (Divine Law).
When we go further, we come across those subjects which are not agreed amongst the Muslims, and there the differences between different sects of Islam begin.

PERSON OF GOD
Sunnis say that Allah has a body, not like the bodies we know. There is a vast material which can be quoted here describing that belief. But as all the Sunnis nowadays are Ash’aritc (followers of Abull-Hassan Al-Ash'ari), I would like to note down his belief on this subject. He says:
"We confess that God is firmly seated on His Throne.....We confess that God has two hands, without asking how.....We confess that God has two eyes, without asking how....We confess that God has a face We confirm that God has a knowledge....We affirm hearing and sight, and do not deny that, as do the Mu'tazila, the Jahmiyya, and the Khawarij....We affirm that God has a power...."
(A.J. Arbery, "Revelation and Reason in Islam", pp.22-23; quoted from "Al-Ibarla" by Abul-Hasan Al-Ash'ari). We, the Shia believe that Allah has not got a body "Verily, Allah is One, unique, nothing is like Him, He is Eternal; Hearing, Seeing, Omniscient, Living, Omnipotent, above every need, He cannot be describe in terms of substance, nor body, nor form, nor accident, nor line, nor surface, nor heaviness, nor lightness, nor colour, nor movement, nor rest, nor time, nor space. He is above all the descriptions which can be applied to His creatures."
"He is away from both extremes: Neither He is just a non-entity (as atheists and in a lesser degree Mu'tazilites implied), nor He is just like other things. He is Existent, not like other existing things". (Shaykh Al-Sadiq, I’ti’qadat’).
Of course, there are some verses in the Qur'an which ascribe the words used for limbs to the person of God. But according to the interpretation of our Imams, they are used in metaphorical, not literal, sense.
For example, the verse:
"Every thing is mortal except His Face" means 'except His Person'. Surely, even the Sunnis cannot say that only the Face of God will remain, while all His so-called limbs will die! Similarly, Allah has used the word ‘Hand’ in several places in the Qur'an. But it means His Power and His Mercy, as in the verse:
"But His Hands are outspread."

CAN ALLAH BE SEEN?

As a direct result of the above mentioned difference, the Sunnis say that Allah can be seen. Some of them, like Imam Ahmad bin Hanbal, say that He can be seen in this world, as well as in the life hereafter. Others say that He can be seen in the life hereafter only.
On the other hand, we, the Shia, say, "That He cannot be seen anywhere, because He has no body, and because Allah says in Qur'an: "Sight cannot reach Him." Surra 6 Verse103 The Sunni brothers use the following verse as their proof:
"Some faces on that day (of judgement) will be fresh, looking towards their Lord." Surah 75 Verse 22-23 But in Arabic language the word "Nadhr" does not imply 'seeing'. Often it is said: "Nadhartu Ilal hilal falam arahu." I looked towards the new moon but I did not see it?
Therefore, the verse cannot imply that they will see God. According to our interpretation, it means that they will be looking forward for the blessings of Allah (swt).

ATTRIBUTES OF ALLAH
According to the Shia, Attributes of Allah (swt) can be put in two distinct groups: First, those Attributes which denote His Person; Second ,
those Attributes which denote His actions:
Shaykh Al-Sadiq says:
"For example, we say that Allah was for ever Hearing, Seeing, Omniscient, Omnipotent, Having power, Living, Self-Existent, One and Eternal. And these are His personal Attributes.
"And we do not say that He was from ever Creating, Doing, Intending, pleased, displeased, Giving sustenance, Speaking; because those Virtues describe His actions; and they are not eternal; it is not allowed to say that Allah was doing all these actions from Eternity. The reason for this distinction is obvious. Actions need an object. If, for example, we say that Allah was giving sustenance from ever, we will have to admit the existence of sustained things from ever. In other words, we will have to admit that the world was from ever. But it is against our belief that nothing except God is Eternal".
It appears that the Sunni brothers have no clear view of this distinction. And they say that all His attributes are Eternal. And that was the actual cause of their belief that Qur'an, being the kalfim (speech) of God, is Eternal, not created. Because they said that He was ‘Mutakallim’ (speaking) from ever.
The Hanbalites so far as said that, ‘Not only were the words and sounds of the Qur'an eternal, so that even its recital was uncreated, but its parchment and binding shared the same qualities....... In the so-called ‘Testament of Abu Hanifa’.......a more moderate view is expressed:
We confess that the Qur'an is the speech of Allah, uncreated, His inspiration, and revelation, not He, yet not other than He, but His real quality, written in the copies, recited by the tongues, The ink, the paper, the writing are created, for they are the work of man." (A.J. Arberry, 'Revelations and in Reason Islam', pp.26-27)
But, as we, the Shia, distinguish between His personal Virtues and His actions. And we say: "Our belief about the Qur'an is that it is The Speech of God, and His Revelation Sent by Him, and His Word and His Book ........ And that Allah is its Creator and its Sender and its Guardian........" (I’tiqadat)
The bitter quarrels, among two groups of Sunnis i.e. the Mu’tazilites and the Ashiariteson on this subject are well-known, and there is no need to relive them.

PLACE OF REASON IN RELIGION
This is one of the most important distinctions between Sunnis on one side, and the Shia on another. To be more exact, I should have used the word Ash'arites, in place of Sunnis. But all Sunnis nowadays are Ash'arites. Muitazilites have become extinct long ago, though some of the great scholars of the recent times like Justice Amir Ali were Muitazilites.
Anyhow, the Shias say: "That irrespective of religious commandments, there is real merit or demerit in different courses of actions, and it is because a certain thing is good that God orders it, and because the other is bad that He forbids it." Sunnis deny this conception. They say that nothing is good or evil in itself. Only what God has commanded us is good and what He has forbidden us is evil. If a thing is forbidden by God it is bad; then if God cancels the first order, and allows it, it will become good, after being bad.
In other words, Shias say that God has forbidden us to tell lie because it is bad; Sunnis say that lie has become bad because God has forbidden it. Shias recognise the relation of cause with effect. Sunnis deny it. They say that there is no cause except Allah. And it is just a habit of Allah that whenever, for example, we drink water He quenches our thirst. Based upon the above difference of attitude about the position of reason in religion are the following differences:
The Shias say, "That God never acts without purpose or aimlessly. All His actions are based on wisdom and intelligent purpose. Proof: Because it is not commendable, rationally, to act without purpose. Sunnis on the other hand, because of their denouncement of rational merit or demerit, say that it is quite possible for God to act aimlessly. "
It follows that, according to the Shias, God does nothing which has inherent demerit in it. Sunnis deny it. The Shias say, "That all actions of Allah are intended for the benefit of His creatures. Because He Himself has no need; and if His actions become devoid of benefits for His creation also, they will become aimless, which is rationally not commendable. Sunnis deny it, because of their stand about rational merit or demerit."

LUTF: GRACE
Based upon the above differences, is the difference about their attitude towards Grace of Allah. The Shias say, "That Grace is incumbent morally upon Allah. By Grace is understood that action on the part of God which would help to bring His creatures nearer to His devotion and obedience and facilitate their moral correction, (which is) morally incumbent on Him."
"Allah Has commanded us to be just, while He Himself treats us with something better, namely Grace Tafaddul."
Sunnis, on the other hand, say, "God leads astray whom He wills and guides aright whom He wills, and it is not incumbent upon God Most High to do that which may be best for the creature". (Creed of al-Nasafi)

GOD'S PROMISES
Based upon our stand on Justice and Grace is our view that:
"Whatever God has promised as reward for a good work, He will fulfil it; but whatever He has threatened as punishment for a bad work it is upon His discretion: If He meted out the punishment, it will be by His Justice; if He forgives it, it will be by His Grace." (Shaykll al-Saduq, 'I'tiqadat') We are confronted both by Kharijites and Mu’tazilites on one side and Ash’arites on other side. Mu'tazilites and Kharijites say, "That it is incumbent upon God to fulfil His threats also. He has no power to forgive."
Ashiarites, on the other hand say, "That it is not incumbent upon Him even to fulfil His promises of rewards." They go so far as to say, "Even if Allah was to send the prophets into Hell, and Satan in Paradise, it is not against virtue, because there is no inherent demerit in any action".

WHY BELIEVE IN GOD?
Shias say: "Man is obliged by his reason to know God, and to obey His commands. In other words, necessity of religion is proved, first of all, by reason."
Sunnis say: Reason has nothing to do with anything. Of course, it is necessary to believe in Allah, but not on account of reason. It is necessary because Allah has ordered us to know Him. According to Shia view, this type of proof creates a vicious circle. Believe in God. Why? Because God has ordered it. But we do not know who is God. Why should we obey Him?

LIMIT OF LAW

The Shias say: "God cannot give us a command beyond our strength, because it is wrong rationally." The Sunni brothers do not agree with it.

OUR ACTIONS: TAQDIR
Are our actions really ours? Or are we just a tool in the hands of Allah? The Shias say: "Taqdir means that Allah possesses foreknowledge of human actions. But He does not compel any man to act in any particular way." To make it clear, it should be explained here, that Man's conditions or actions are of two kinds actions:
1) Those actions about which he can be advised, ordered, praised or blamed. Such actions are within his power and are independent upon his will.
2) Such conditions about which he cannot be praised or blamed, like life, death, etc. Such conditions are outside his sphere of will or power. For example, we can advise a patient to consult this or that doctor and remain under his treatment; but we cannot advise him to become cured.
Why this difference? Because getting treatment is under his power, but getting cured is not in his power. it is something which comes from Allah. But even our freedom of action is a gift of Allah. He has given us the power, the freedom, the strength, the limbs, the wisdom and every thing with which we do any work.
Therefore, we are not independent of Allah, because our freedom is not only given but even sustained by Him. But our actions are not compelled by God, because He, after showing us the right and wrong ways, and after enjoining us to do right, has left us to our own free will. If we go wrong, it is our own choice. Shaykh al-daduq says: "Our belief in this respect is what has been taught by Imam Ja'far al-Sadiq, 'There is no compulsion (by God) and no relinquishing the authority (of God); but a condition between these two conditions'.
Then Imam was asked. 'How is it?' He said. 'Suppose you see a man intending to commit a sin; and you forbade him; but he did not listen to you; and you left him; and he did commit that sin. Now when he did not heed to you and you left him, it cannot be said that you ordered him or allowed him to sin." I’tiqadat In other words, we believe that God has given us power and will and then has left us free to do what we like. At the same time, He has taught us, through the Prophets, what is right and what is wrong. Now, as He is Omniscient, He knows what will be our actions in different times of our life. But this knowledge does not make Him responsible for our actions more that a meteorologist can be responsible for cyclones and storms, if his forecasts come true. True forecasts are the result, not the cause, of the impending event.
The Sunnis on the other hand say that Allah is the creator of all our acts. "No act of any individual, even though it be done purely for his benefit is independent of the will of Allah for its existence; and there does not occur in either in physical or extra-terrestrial world the wink of an eye, the hint of a thought, or the most sudden glance, except by the degree of Allah ..........Of His power, desire and will. This includes evil and good, benefit and hurt, success and failure, sin and righteousness, obedience and disobedience, polytheism and belief." (al-Ghazali: as quoted in 'Shia of Irldia' p.43)

PROPHETHOOD
Based upon their belief of lutf (Grace), the Shias believe that it is incumbent upon Allah to send Prophets or their successors in this world to put people on right path. Sunnis say that it is not incumbent upon Allah, because they do not accept necessity of lutf. Shias and Sunnis in first instance, and then Sunnis among themselves, disagree about the theory of 'ismah (sinlessness) of the prophets.

SINLESNESS
What is our conception of Ismah? It is lutf (Grace) of Allah which helps a person to refrain from sins, without effecting in any way his will and power. A Ma'sum (sinless) person as power to commit sins; but he does not even think about sins; because his spiritual standard is so high that such inferior things do not enter his mind. However Sunnis do not speak with one voice upon this subject:
The First difference: Is about the point when Ismah begins. Some say it is after the declaration of Prophethood; and others say that it is since childhood. Second Difference: Scope of Ismah before declaration of Prophethood: Some say that it covers all sins; majority says that they are protected from kufr (infidelity) only.
Third Difference: Scope of Ismah after declaration of Prophethood; it is agreed that the Prophets could not tell a lie after Prophethood. But what about other sins? Some say that they could commit other sins either intentionally or unintentionally; but the majority says that they could commit it unintentionally, but not intentionally.
The Fourth Difference: About minor sins: They say it was possible for the prophets to commit minor sins, even intentionally. But that they were protected from such minor sins which might have degraded them in the esteem of people.
The Shia stand about Ismah is that all the prophets were sinless and infallible; they could not commit any sin, whether capital or minor, and whether intentionally or unintentionally; and that they were ma'sum from the beginning of their life till their last breath. The Prophets:
Shaykh al-Saduq says about the prophets that: "Their word is the word of God, their order is the order of God, their forbidding is the forbidding by God And that the Chiefs of the prophet are five and they are (called) Ulul’azm - and they are:
# Noah
# Abraham
# Moses
# Jesus
# Mohammad

Be the Blessings of Allah upon them all! And that The Holy Prophet Mohammad(pbuh&hf) is their Chief and the best of them all." Imams:
The Shias say, "That Imam must be appointed by God; that appointment may be known through the declaration of The Prophet or the proceeding Imam." Sunnis say that Imam (or Caliph, as they prefer to say) can be either elected, or nominated by the proceeding Caliph, or selected by a committee, or may acquire power through a military coup. If he is elected, it is enough that one man should do bay'ah (allegiance) to him.
The Shias say: "That Imam must be Ma'sum (sinless)." Sunnis say (including Muitazilites) that Ismah is not a condition for caliphate. Even if he is tyrant and sunk in sins, Hanbalites, Shafi'ites and Malikites forbid people to rise against that Caliph. They say that they should persevere.
The Shias say, "That Imam must possess above all such qualities as knowledge, bravery, justice, wisdom, piety, love of God etc." Sunnis say it is not necessary. A person inferior in these qualities may be elected in preference to a person having all these qualities of superior degree.
The Shias say, "That Ali Ibin Abe Taleb(as) was appointed by Allah(swt) to be the successor of The Prophet(pbuh&hf), and that The Prophet(pbuh&hf) had declared it on several occasions. The Sunni brothers say that The Prophet did not appoint anybody to be his successor.