Christianity is the religion (or set of religions) that encourages adherence to the teachings of Jesus Christ. But since Christianity branches into many different categories, you must remember that the idea of "Christianity" is different from Christian to Christian.
Fundamentalist Christianity, which BornAgainChristian is part of, requires adherence to the "five fundamentals," characteristically a literal interpretation of the Bible.
Liberal Christianity holds that some of the Bible is merely myth or metaphor and not to be taken literally.
Catholic Christianity requires observance within the structure of the catholic church, built in a hierarchy of priests, bishops, cardinals, the Pope, and God. The meaning of the Bible is given through the church.
There are also a number of Chrstian "cults" including Mormonism and Jehovah's Witnesses who have exclusive brands of Christianity.
The thing that all Christians have in common is a belief in and an adherence to the figure of Jesus Christ as portrayed in the New Testament.