Christianity is a set of religious beliefs and practices that generally center around the person of Jesus Christ, a figure who is supposed to have existed in first-century Judea, and who, according to the story in the New Testament of the Bible, was sent by God to Earth to be punished vicariously for original sin by being crucified. Among the more improbable things attributed to Jesus are the converting of water into wine, walking on water, healing through miraculous means and being conceived asexually inside the womb of Mary.
Christianity is split into a variety of branches - Catholicism, Protestantism and Orthodox Christianity. Each of these has sub-divisons within them. Some of the churches which preach Christianity do it at a very extreme or all-encompassing level - often, these people are called Fundamentalists (holding, as they do to what they think of as the fundamental tenets of Christianity, as laid out in a book called The Fundamentals). Evangelicals are a similar in their theology to many of the Fundamentalists, believing that God has called them to spread their message to others, and placing heavy prominence on the need for a personal conversion (known as being born again). There are also very liberal Christian denominations, ranging from cultural Christians and Unitarians through to promoters of the social gospel.
Specifically for arguments against Christianity, see:
Last updated by Nelson Feb 7, 2009.