The idea of BuddyPress is to take a standard vanilla installation of WordPress MU and turn it into something that represents more of a community building tool, or niche social network.

BuddyPress is essentially a set of WordPress MU specific plugins. Each plugin adds a distinct feature (or component) to BuddyPress and only handles functionality for that specific component (for example, private messaging). BuddyPress also has a core plugin that all other plugins require, it contains shared functions and performs the basic modifications to the WordPress MU interface.