Your rotors are warped. Replace the brake pads and have the rotors resurfaced or replaced. Make sure the wheel lug nuts are torqued to specs when the tires/wheels are reinstalled. Over tightening the lug nuts will warp the rotors.
ANOTHER ANSWERThe above is absolutely correct, however, heat is perhaps the most common cause of warped rotors. If your vehicle has to stop when there are heavy loads, or if you "ride the brakes" (one foot on the brake pedal while you're driving), or if you have a worn caliper that's constantly putting a little pressure on one of the rotors, there can be a lot of heat generated which will result in a warped rotor.
On the other hand, if you've had the brakes replaced, and the PREVIOUS mechanic used a cheap replacement rotor, that may be the cause. Cheap rotors warp easily.
