Some people define themselves by their religion. Some people die for their religion. Many more kill for it. It's a good idea to have a grasp of how a person (or culture) is affected by their cosmology.
General, "Survey" Websites
The Religious Tolerance page covers dozens of different religions, with many links if you want to know more.
Religious Movements Homepage from the University of Virginia.
A site that explores the "big five" faiths.
Finally, the home page for the Philosophy and Religion internet course taught at Lake City Community College.
Specific to Major Religions
The Catholic Encyclopedia
A page on Monasticism in Britain.
"Alternative" Belief Systems
The Witches Voice
Pagan Culture Center
A couple of looks at Native American spirituality: From the religious
tolerance site and the religious