Roll bars only exist behind the drivers seat. And can be welded or bolted in.

Cage goes in front and behind drivers seat.

Autopower roll cages bolt in, go in front and behind drivers seat, but are still a roll *cage*.

 

^^^I agree with DriveFast7, but to expand, obviously since I'm bored and listening to Tetris remixes.
Yeah, Cages are just that, a cage that encompasses the driver all around. Some even go into the Engine bay to prevent the engine from being harmed in a frontal collision.

Rollbars are just designed to be behind the driver and prevent the roof from collapsing and allowing a safe usage of harnesses without having to get a full cage. A full cage would require a helmet and that wouldn't be streetable.