Junior Mod = less than six months of "service", restricted to moderating their specifically-assigned forums. Still "learning the ropes".
Moderator = less than two years "service". Have the ability to moderate any forum they can see. (Not all mods can see all forums, but every forum is visible to *some* mod - and admins, of course.) The "workhorse" of the system. While encouraged to focus primarily on their assigned forums, they are able to handle problems pretty much anywhere in the forums, if necessary.
Senior Mod = more than two years "service". Same abilities as regular Mods, but they are recognized for their longevity in this demanding field. They provide additional guidance for newer mods, sometimes acting as mentors. Their extensive experience gives them a better grasp of the "big picture".