WordCamps come in all different flavors, based on the local communities that produce them, but in general, WordCamps include sessions on how to use WordPress more effectively, beginning plugin and theme development, advanced techniques, security, etc. To get an idea of the types of sessions typically seen at WordCamps, check out the WordCamp channel at WordPress.tv.
WordCamps are attended by people ranging from blogging newbies to professional WordPress developers and consultants, and usually, combine scheduled programming with unconference sessions and other activities. For more information on common WordCamp elements, read about what to expect at a WordCamp.
The first WordCamp was organized in San Francisco by Matt Mullenweg in 2006, and since then local communities around the world have organized hundreds of others.