You:
a. get your tuition waived. So, you pay no tuition. Some people have trouble getting this 😛 My tuition bill says $11000.00 each semester, then goes to $0.00 when they process the waiver. Typically waivers are for a certain number of credit hours per semester, and that changes based on your assistanceship load. So, a 1/2 time TA might get 6 credits waived, while a Fellow without teaching or research obligations might get 12. You might still pay some pittance of fees.
b. get money for going to school. You'd be a teaching assistant (TA), a research assistant (RA), or (best) a Fellow. The amount you get varies wildly. I've heard as low as 10k and as high as 30k, and certain special circumstances (e.g. being in the military) can get you more money than that.
