Linehaul is hauling loads from terminal to terminal or to a half/way meet, and not touching any freight.
Extraboard is LH, only you stay out all week. Sometimes you get home during the week, but that's according to the freight levels.
P&D is city- you pick up and deliver, and bring back the freight to the terminal.
Hub is like a combination driver/dock worker. I don't know much about that category, other than if I'm not mistaken, their pay rate is lower (for doing more work...hmmm.)
Utility is a new category that was started a few years ago, and from what I understand, it is whatever they need you to do that week; could be driving, dock, jockeying, etc.
According to Mildew, extraboard is the bees-knees, but don't pay him much attention. There's a good chance he's drunk-posting again! (Love ya Dew!) Actually- it is what you're willing to put in to it. Generally, if you want to bust yer donkey, there are dispatchers who are sadistic enough to help you do just that!