Shia and Sunni are terms for the two largest groups of Muslims. The points of convergence between these groups are a lot more than what they differ in, what is important here is that the points of differences cannot be understood without returning to the reason or the Islamic thought that the Shiite school is based on in comparison to the other schools.
The Great Martyr Al Sadr explains in his book ''The Emergence of Shiism and the Shiites'' speaking about the genesis of the two main currents during the Prophets (s.a.w) lifetime:
''The two chief tendencies closely associated, from the start, with the emergence of the Islamic Ummah during the Prophets lifetime are:
One: The current representing a belief in the devotional acts of religion, it's arbitral power and the unconditional acceptance of religious stipulations for every aspect of life. (This is the tendency of the school of those who upheld the rights of the Prophetic Household and Shiism.)
Two: The current, which sees religious faith as eliciting devotional deeds within the special scope of overt acts of worship only. It believes in the possibility of independent legal judgment (ijtihad) and free discretion for the amendment and improvement of religious stipulations according to benefits (masalih), which might accrue in other domains of life (This is the tendency of the Sunni Schools.)
These two currents were both reflected and manifested when the Prophetic tradition granting the leadership to Ali Ibn abi Talib was confronted, one strived in opposition to the tradition while the other stood with the tradition.
From then the school of obeying the tradition by following Ali was born. He then goes on to explain: ''The two currents, whose rivalry began in earnest during the Prophets (s.a.w) own lifetime, were reflected in the Muslims position regarding the thesis of the Imams' pre-eminence in the mission after the Prophet (s.a.w).
Those representing the devotional tendency (as opposed to the one for independent legal judgment) found in the Prophets (s.a.w) stipulation the reason for accepting the doctrine without hesitation or adjustment.
The advocacy of independent legal judgement was viewed as offering the possibility of reason from the pattern established by the Prophet (saw) whenever judgement imagined to be more harmonious with the circumstances was called for. By the same token one observes that Shiites arose immediately after the Prophet's (s.a.w) death, representing the Muslims who adhered in practice to the doctrine of the Imams'
pre-eminence and leadership, the first steps of whose implementation the Prophet (s.a.w) had declared obligatory right after his departure. The Shiite current embodied from the first a repudiation of the Saqifah Council's attempt to paralyse the doctrine for Imam Ali's pre-eminence and to transfer authority to someone else.
("The emergence of Shiism and the Shiites." by Shaheed Al-Sadr.)
Shaheed Al-Sadr believes that the Muslims did not follow the leadership of Imam Ali and the reason for this goes back to their way of thinking.
On these bases the Shiite viewed the position of Imamate from a special perspective emerging from the importance of leadership and that the Imam is the true caliph of the Prophet (s.a.w) on earth.
Therefore the Imam had to be of impeccable character & outstanding qualities enabling him to occupy the position of leadership.
Like infallibility, endowed knowledge and complete understanding of the existence man and the divine law. Thus the Imam (as) is a reflection of the Prophet (s.a.w) in all his qualities except the connection with revelation.
"You are to me in status as Haroon (as) was to Musa (as) except that there is no Prophet after me." (Prophetic tradition)
This is the basic difference between Shiite and Sunni, that the Shiite understand the position of leadership to be absolute and not relevant, that it could be through the allegiance of the Ummah or its consensus on a specific person as is the view of Scholars of the Ahle-Sunnat.
Therefore we find that the primary and secondary aspects, which both schools have practiced according to the dictates of the Prophet (saw) like prayer, fasting, pilgrimage, other acts of worship and transactions in their general forms have not been subject to differences.
Differences however occurred in certain analytical deductions thus on one hand for example the Shiite differ with the Hanabila but concur with the Shawafi in a specific issue. Similarly the Shawafi will concur with Hanabila based on proof, likewise with the other schools (Maliki and Hanafi). We find on the other hand they (the Shiite) concur and do not differ with the other Sunni Schools in most issues. For further information on this topic you may refer to ("Figh according to the Five Schools") Sheikh Jawad Mughniyya.
The same applies to the theological aspects, we find that the general principles that makes a person a Muslim or a Kaafir viz. Unity of Allah, Prophet-hood and Resurrection are agreed upon, but difference in Imamate which the Shiite consider to be of the fundamentals whereas the Sunni's don't.
On the issue of Justice we do not find the difference based on Sunni or Shiite but rather it is between the School of Adliyat (proponents for) and the School of Asha'irat (those against).
The Mu'tazila for example are part of the Sunni School but are proponents for the concept of Divine Justice. The issue of Justice is dealt with in theological discourses.
Like the issue of Good and Evil being of the intellect or not.
Similarly you find that the Shiite concur with the Asha'ira in some theological issues and differ with the Mu'tazila, thus the issue was not viewed on the basis of
Jurisprudence but studied according to intellectual and legal means.