I have a 98 Hyundai Tiburon, the front brakes work gr3eat but the rear don't, what could be the problem?

Does it have rear drum brakes? It could be that they are out of adjustment. How are you judging their performance...by the parking brake? Or with the actual brake pedal...cause the parking brake is a manual, cable-operated engagement whereas the brakes themselves are hydraulic. There's a Brake Proportioning Valve between the front and the rear also that reduces the pressure going to the rear calipers or wheel cylinders..but in my experience when the prop valve goes, the rears start to skid and lock-up, not go inoperative.

Has disk brakes on the rear, when you drive it and hit the break pedal, the rear brakes don't work at all

No fluid loss in the master cylinder? Lift the back of it up and have someone apply the brakes while you try to spin the rear wheels, to see if they're getting anything *at all*....a Proportioning Valve is likely going to be your cheapest fix if you go at it from the "throw parts at it til it's fixed" angle...which I usually try to avoid.