WordPress Multisite adds a new role to the list: Super Admin. Super Admins have the ability to administer the entire Multisite network.
User management in Multisite is similar to user management in "vanilla" WordPress. When a new user is added to a given site, s/he can then be added to other sites in the network, with different capabilities. For instance, s/he can be an Administrator on Site A, an Editor on Sites B and C, a Subscriber on Site D, and have no particular privileges at all on Sites E through X.