WordCamps are casual, locally-organized conferences covering everything related to WordPress, the free and open source personal publishing software that powers over 75 million sites on the web.
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.