Answer
The Isma'ilis are a branch of Islam that separated from mainstream Shia due to the crisis of succession after the death of Ja'far al-Sadiq. They believe that Isma'il ibn Ja'far was the rightful successor to Ja'far, and that the Imamate passed to him and his descendants. The mainstream Shia believe that Musa al-Kazim was the rightful successor, and that the Imamate passed to him and his descendants.