A lot. A GLS is just a regular Jetta. It's usually a 1.8t but can be cloth, leather etc. It looks like pretty much every jetta you see. A GLI(04.5+) is a 1.8 turbo, has 18 inch BBS wheels, Recaro Seats, painted front and rear lip, side skirts, 6 speed transmission. A 01-03 GLI also is 6 speed, but its a VR6 engine, and it looks like pretty much any old jetta with a few extra standard features.
(I am assuming you are talking Mk4's.)