WordPress MU

I have the following user roles available but I do not know the functional differences that distinguish them.

Administrator
Editor
Author
Contributor
Subscriber
Keymaster
Moderator
Member
Inactive
Blocked

Some are self explanatory or intuitive but others appear to have no differences. I tested the dashboard as both Keymaster and Editor and I saw no difference between the two. What also is the difference and functions allowed for subscriber VS member?

I've searched but could not find an online resource that explained the user roles.

I also have another problem. When users sign up to our site, there is no role assigned. Is this a sign up option I can change? I never saw this in WP.

Thanks!

You've got bbpress roles listed in there too. Are you running buddypress?

User roles in WP & MU:
http://codex.wordpress.org/Roles_and_Capabilities

The only other role MU adds is a Site Admin as a superuser who can get in everywhere and do everything.

I am using BuddyPress and that appears to be what is going on.
Can the master blog only have one site admin?

site admin is across the site versus a specific blog. So, a site admin had administrator permissions on all blogs.

you can add additional 'site admins' via the Site Admin->Options settings, but note that this gives them privileges for ALL blogs as wpmuguru points out

"Can the master blog only have one site admin? "

That's what it does by default. Other people who sign up do not get to be site admins unless you specify. They can only control their own profile, and their own blog (if you let them have a blog).