ChristianityIt's a nice word, much better than "Christism" or "Jesusism" (see Jesus Christ).
In the United States, almost every cult claims that their church is "Christian": the Moonies, the Mormons, Christian Scientists, Jehovah's Witnesses... it's just an easier sale that way. Even the Scientology logo is a modified crucifix.
Christians are, generally speaking, the 2 billion people who claim in one way or another to be following the teaching of Jesus Christ. Most (but not all) of these believe that Jesus was divine in some way. That number doesn't include Muslims, another 1.3 billion people who believe Jesus was an important prophet but not the son of God.
With that broad label of "Christian" are hundreds of secondary classifications, based on differences of opinion about the technicalities of who Jesus was, how he came to be, cosmology, theology, morality, which the books of the Bible are OK and which are not, and how those books should be translated.
For a religion based primarily on the credo "Do unto others as you would have them do unto you," there is an awful lot of hostility between adherents of these differing interpretations.