Rastafarianism is a religion that had its beginnings in Jamaica in the 1930s. It is essentially part of the Judaeo-Christian tradition, but one with an Afrocentric flavor.
Rastafarians believe in the Judaeo-Christian God, whom they call Jah. They believe in the divinity of Jesus Christ. But they also believe that the Emperor of Ethiopia, Haile Selassie, was the second coming of Jesus. That is where they get their name from--the Ethiopian title "Ras" and Selassie's given name, Tafari.
Rastafarians do not have an organized church with specific doctrines. However, they tend to focus closely on the Old Testament, which is why, for example, Rastafarian men do not cut their hair and simply wear it in dreadlocks. Rastafarians are most famous in Western eyes for their use of marijuana as an aid to spiritual insight and for their close connection to reggae music, starting with Bob Marley.