WordPress MU, or WordPressµ, is the multi-user version of the famous WordPress blogging application. It is ideal for people who want to set up a large network of blogs.
Plugins and themes are controlled by a central system administrator. Individual themes can be made available to all blogs or just to specific ones. So unlike the single-user version of WordPress, individual MU bloggers cannot upload their own plugins and themes, only select from the ones made available by the system administrator.