Mahmood Abu Maryam, Trying to make sense of it all... Answered 4 years ago

Both Sunni and Shi'a muslims are the ahl al-qibla and belong to the ummah of Prophet Muhammad (s). Their main difference is considered to be in the matter of succession to the Prophet (s) where the Shi'a believe that Imam 'Ali b. Abi Talib was appointed by the Prophet (s) to be his successor, whereas the Sunni's believe the matter was not determined by him (s) and the caliphate of Abu Bakr was legitimate.

Both groups refer to the same Qur'an as their primary source of Divine instruction and guidance.

The Shi'a adhere to the family - ahl al-bayt - of the Prophet (s) in matters of understanding the creed and law of Islam and the sunnah of the Prophet (s), whereas the Sunni's refer to the Companions of the Prophet (s) to understand religion.

Many early Sunni scholars of jurisprudence directly and indirectly benefited from the teachings of the Shi'a Imams, in particular Imam Ja'far al-Sadiq, and that resulted in significant consistency between the rulings found in the Sunni schools of law and the Ja'fari Shi'a school of law. Of course, there remain areas of difference as well.

Here is a great book to check out the similarities and differences from the jurisprudential angle:

The Five Schools of Islamic Law