WordCamp is a conference type of event that focuses squarely on everything WordPress. Everyone from casual end users all the way up to core developers show up to these events. These events are usually highlighted by speeches or keynotes by various people.
The official annual WordCamp (http://central.wordcamp.org) was held on August 16, 2008 in San Francisco. Regional WordCamps are happening around the world in places like Cape Town, Manila, Beijing, Portland, Salt Lake City, Birmingham, Raleigh-Durham, Toronto, Honolulu, Sydney, Philippines, and Washington D.C.