WordPress Multi-User, better known as WordPress MU, has existed for years as an offshoot of WordPress. WordPress 3.0 merged all of the MU functionality into the core of WordPress, which means that you can now run multiple sites from one installation of WordPress. This is particularly useful for BuddyPress community-driven sites, as you may want to give each of your users their own website. You can manage themes, plugins, upgrades and users all from the backend of your primary WordPress site.
How to enable BuddyPress in WordPress 3.0
In WPMU, there was an option that would let you add Buddypress function on all multi-blogs (network blogs) installed under the main installation. Due to the big merge in WordPress 3.0, now you can enable BuddyPress in WordPress using the wp-config.php file as well.