Those are all your roles that other plugins have likely generated. WPBB only creates one role and that is one called "Guest" so that you can set permissions for guests. It just displays all the rest of your roles so that you can set permissions for them, too. You could try from your admin dashboard -> WPBB -> Tools -> Refresh Roles to remove any roles which you may not be using anymore but which haven't been removed properly.
As for the 'author' and 'contributor' roles you mentioned...
These are WordPress's default roles:
Subscriber (default role for new users)
Contributor (can comment on posts)
Author (can create own posts)
Editor (can edit others posts)
Administrator (the boss)
:) Their exact capabilities depend on any plugins you have installed but that's the main default difference.