If you've ever had to execute an emergency stop in a car with antilock brakes, then you're familiar with the type of rapid brake-pedal pulsing that comes from the quick grabs the system applies to the rotor to slow the car. However, if your brake pedal pulses in this way under normal braking circumstances, you could have a problem.
Generally, a vibrating brake pedal indicates warped rotors. Their uneven surfaces will thrum against the brake pads, and you'll feel the feedback through the brake pedal.
Rotors usually only warp when they're under extreme stress for an extended period. The friction-generated heat caused by driving down steep mountainsides or by stopping frequently while towing something heavy, for example, can cause the metal of the rotors to change shape.
If you haven't stressed your brakes recently, but you still feel vibration in the pedal, you may have misaligned wheels. In either case, it's best to see a mechanic for diagnosis.