Social networking in a box, BuddyPress is a social networking plugin for WordPress.
BuddyPress was conceived in 2008 while working to add social networking features to a WordPress MU powered site. The first official stable release was in May 2009. The platform has grown and morphed considerably since then, into the dynamic, easily extensible package you see today.
Just like its parent project WordPress, BuddyPress is a completely open source endeavor. Everything from the core code, to the documentation, themes and plugin extensions are all built by the BuddyPress community. This means anyone can help the project by contributing their time and knowledge.