Shi'ites share the same Qur'an, Prophet, and general beliefs as the Sunni Muslims. There is, however, a difference in emphasis and in the order of precedence or ranking of these fundamental beliefs and duties. This difference in precedence produces subtle and yet deep shades of "coloring" that permeate the Muslim's attitude, behaviour, and actions. The philosophical foundation (the coloring) of the belief thus shifts slightly one way or the other and therefore what is built upon the foundation shifts as well.
The difference of emphasis can be seen in the ranking of the basics of the creed by Shi'ites. The usual (non-Shi'ite) ranking focuses primarily on action - on acting out the basics of the religion. This is summarized in the well known five pillars of Islam: The shahadah (the declaration of faith - that there is no God but Allah and that Muhammad (s.a.) is his Prophet). Prayer (a minimum of 5 prayers each day) Hajj (the once (at least) in a lifetime pilgrimage to Mecca) Fasting Zakat (a religious tax to ensure the welfare of the needy)
The Shi'ites obviously have no differences with the above pillars but they first lay a foundation upon which the pillars stand - in other words their emphasis is placed first on theological / philosophical (as derived from the Qur'an) principles and then the active dimension of the religion rests upon that foundation and in fact derives and emerges from that very foundation. The pillars are not free-standing pillars (that would be pointless - ask any architect) but rest on a foundation and support a larger structure. The Shi'ites begin with a somewhat different list of five fundamentals: Tauhid (The principle of God's Uniqueness and Unity - the first part of the Shahadah would naturally emerge from this) Adl or Divine Justice (The principle of God's justice - the remaining principles flow out of these first two principles) Prophethood (the principle of the nature of Prophethood and the finality of Muhammad's (s.a.) Prophethood - the second part of the shahadah would naturally emerge from this) Imamate (the principle of the means of guidance for mankind after the time of the last Prophet - this would be a point of divergence between Shi'ite and other Muslims - for Shi'ites the Imamate is a spiritual office first and a political office only secondarily). Eschatology (the principles relating to what happens after death and with the judgment and resurrection)
These principles then form the foundation of the acts of worship and the ethics of Islam which all Muslims are obliged to observe. The acts of worship are: Prayer Fasting Hajj Religious taxes, including Zakat and Khums (a one-fifth tax on surplus income and certain luxury items) Amr b'il maruf wa nahy an al-munkar (Directing towards good and directing away from evil - Jihad is included under this heading)
The dictates of ethics are called Tazkiya (Refining of the soul (self)) and (in a brief summary form) are as follows: Tazkiya of thought (striving for knowledge and avoiding mental states which weaken knowledge, such as: negligence, prejudice, concealment of the truth, domination of the passions, reliance on supposition etc.) Taqwa (achieving a true and constant consciousness of God) Ikhlas (seeking God's pleasure - cultivating the most beautiful behaviour for God's sake alone since obedience without love may lead to hypocrisy) Tawakkul (reliance on God) Sabr (patience, especially in the face of difficulty) Shukr (giving of thanks to God) Infaq (generosity) There are many other ethical qualities which fall under the general headings listed above.
While most Muslims would follow much of the above, once again, the difference is primarily one of emphasis, and of precedence and ranking. For Shi'ites it is an article of Faith to be fully conscious of all of the principles, otherwise the actions can degenerate into mere ritual or formality and the result would be a society where the individual members carry out all these actions and their impact and significance is almost non-existent. In other words, the actions alone won't bring about reform in an individual and in a society and in the world. The actions must be founded upon comprehensive principles that are well understood. Only then can one's intention be truly pure and clear. And once this happens the actions will bring about profound change as they did in the Prophet's time.
The single greatest distinction is the concept of the Imamate. For Shi'ites the most important function after the death of the Prophet(s.a.) was the clarification of Islamic teachings and the preservation of the Sunna of the Prophet(s.a.). The Shi'ites look to the Ahl al-bait (the household of the Prophet(s.a.)) and the Imams for the complete and perfect understanding of the sunna and as models and examples of that sunna being put into practise. And after the Prophet(s.a.), they (the Imams) were the "keepers of the book", the explainers of the Qur'an, since the Prophet had taught them from his own knowledge far beyond the extent that he taught any other companions. He was Ali's personal teacher on a daily basis from the day of the declaration of his Prophethood(s.a.) to the day he died.
It is a simple matter that there was no one else as qualified in knowledge or in personal behaviour as the Ahl al-bait and the Imams. They were the best means of preserving and conveying correctly the sunna of the Prophet (s.a.). So the Imams are like Islamic technicians - they understand Islam and they take care of it. They do not merely hear it and pass it on. For the Shi'ites, the Imams are the preservers of the pure Islam of the Prophet (s.a.) and they are followed whether they hold political power or not.
The Prophet(s.a.) had asked Muslims to look to Ali for the correct leadership so the Shi'ites did just that whether Ali was in a position of political power or not - Political and social leadership was only a small part of Imamate - its main function was the preservation of the purity of Islam (its goal was a long term goal that had to survive the squabbles for power and empire that swept the Muslim world from the time of the Umayyads onwards and to preserve Islam from the distortions created by the Umayyads and other succeeding empires).