Well, you are right that these things, which are... umm... stylized stars... showcase the "rank" of a member. They correspond to a user's title. Mostly, it is post-based, but quite often you will find members with special positions, be it access to the teens area, the girls-only club or just early full access, which is usually granted if we know the person from another place or after some personal talk.
Specifically, any differently-colored star represents a special group. As of now, five green stars symbolize that the person is moderator for the forum you are viewing, while global moderators who are not directly responsible for that board will have six blue stars. Four red stars symbolize an administrator. Not every staff member actively displays his rank, though, for instance Stuart is an admin without any special group.