Concept of Users


I am just starting with WP Mu. Please pardon my basic questions.

For now, I am wondering the concept of Users in a multi site environment. How does this work? Say I have ten blogs. Do each of the blog have their own users? Do the users of one blog have to register again to see the rest of the blogs? Can one blog owner require its own set of users and opt out of the system wide users? I am highly confused. Any help will be appreciated.