Wikipedia has a useful article on contracts too: http://en.wikipedia.org/wiki/Contract_%28BDSM%29

On the whole, Wiki has a great deal of BDSM-related discussion. It's not an undisputed authority in any way, but at least most of these articles have been written by people within the BDSM scene who know what they are talking about:

http://en.wikipedia.org/wiki/List_of_BDSM_topics