BuddyPress can be defined as a set of plugins that are used to turn WordPress MU into a social networking platform. In other words: it will let you use the many features we all ready associate with WordPress, but adding a whole social element to the mixture.
Each plugin adds a social feature of its own, and there is also a core plugin which is necessary to install in order to employ the different plugins as it contains shared functions that modify and enhance the WordPress MU interface. The advantage of having separate plugins is that you can handpick the ones that are incumbent to you and forget about setting up elements that you might not need, and that might even cause maintenance niggles. For example, if you want to add only a messaging functionality you can do that, and add just that element to the equation. On the other hand, you can create a whole social network from scratch if you have the skill and the desire to do so.
This project is constantly being updated, and you can always procure the latest downloads at the site. At the end of the day, it is safe to say that the many WP adepts are going to find the site at least worth checking out, and this appeal will only broaden as the project evolves.
BuddyPress.org In Their Own Words
“BuddyPress is essentially a set of WordPress MU specific plugins. Each plugin component adds a distinct feature 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.”
Why BuddyPress.org It Might Be A Killer
WordPress coupled with full social networking features? If that is not killer enough, I don’t know what comes close.
Some Questions About BuddyPress.org
When will the project be finished? What issues should be dealt with first in order to improve it? 
