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Shi'a Islam is the second largest branch of Islam (the largest being Sunni). About 20% of Muslims are Shi'a overall. It is the dominant form of Islam in Iran, Iraq, Bahrain, and Azerbaijan. Shi'as are subdivided into Twelvers (Imamiyya), Ismaili and Fivers (Zaidi).
 Differences from Sunni Islam
Shiites believe that the rightful successor of Muhammad was his cousin Ali, rather than his father-in-law Abu Bakr. They also differ on the identity of the Mahdi. Twelver Shiites believe that the Mufi was Muhammad al-Mahdi who was "hidden" in the late 800s and will return someday; other Shiite communities believe that an earlier imam was in fact the Mahdi.
Due to the difference in Hadith collections, the split of the Shi'a represents variations in non-Quranic law, which in Sunni Islam is often derived from scholarly jurisprudence. Depending on further differences between Shi'a subdivisions, the Sunni-Shi'a differences are either exacerbated or mitigated depending on the rationale used in the jurisprudence of law for each respective subdivision.
Aside from the Imamate bloodline, role of the Shi'a Imam, and the belief in the Occultation (or "hiding" of the Mahdi), it can be argued that jurisprudence of law differences between the mainstream representatives of each Shi'a subdivision are, in fact, rather small.
Shi'a Islam, unlike Sunni, allows for temporary marriages of a contracted amount of time called nikah mut‘ah/segheh; they can be contracted from as little as a day to up to two years. 
Theological matters may show more variance, as the historical debate between Mutazili or Ashari theology applies differently to each group. Mainstream Sunnism almost ubiquitously represents the Ashari theological line, while the Sevener Shi'a show the most Mutazili influence. Theological differences are apparent in issues including the characteristics of God, the matter of free will, the ability to rationalize the word of God, and the relationship between sin and faith.