He was best known as a billionaire philanthropist, who invested his wealth in yachts and jets as much as he did in the Aga Khan Development Network, a gigantic foundation that says it has 96,000 employees worldwide and funds development programs, notably in Asia and Africa. Karim Al-Hussaini, also known as the Aga Khan, died on Tuesday, February 4, in Lisbon, aged 88.

He had the ear of almost all of the world's leaders – and also regularly made the front pages of celebrity magazines – and was, above all, the imam of the Nizari Ismaili faith, an esoteric branch of Shia Islam which, despite the prominent role it played in the Middle East's history, has always been the source of many legends and fantasies and remains largely unknown. Here are five keys to better understanding the religious tradition.

Ismaili Islam is a branch of Shia Islam. According to the Shiites, temporal and spiritual power is vested in the imams, or those who lead their followers from "in front," who are appointed from among the descendants of Fatima, daughter of the Prophet Muhammad, and Ali, his son-in-law. The Ismaili branch, for its part, broke away from the other branches of Shia Islam around the year 765.

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