WordPress MU, or multi-user, is designed to runn hundreds of thousands of blogs with a single install.
You can pronounciation like wordpress mu(EM-YEW) or you can aslo call “WordPress Mooooo”

Who are using WordPress MU?
•    Newspapers and magazines.
•    Big Universities
•    Blog networks
•    Niche-specific blog hosts
•    WordPress.com uses WordPress MU!!
Features of WordPress MU.

Code which differs on wordpress mu side is that is bootstrap files, and administration for multiple blogs and users. Most of the basic code is same for both. I can say 95-99% that is amazing.
Wordpress mu can have multiple authors and you can assign different roles.

Most of the plug-in work just like in regular WordPress because basic frame of the code is same but of course not all.
But some that modify core tables or create tables of their own in the DB might have difficulties, depending on how they’re coded.

Worpress Mu can be configured on sub-domains or top level domain, just like regular wordpress.

You can enable Akismet spam protection same like your favorite one!!  But Developer of the wordpress mu prefer to go for Pro-blogger API key.

WordPress MU creates tables for each blog, which is the system we found worked best for plugin compatibility and scaling after lots of testing and trial and error. This takes advantage of existing OS-level and MySQL query caches and also makes it infinitely easier to segment user data, which is what all services that grow beyond a single box eventually have to do.
Lastly you can ask can I develop wordpress mu so the answer is pretty simple! YES because if you are familer with wordpress you will feel pretty comfortable with WordPress Mooooo!!