Rotors are key components of a car's braking system. When you press the brake pedal, fluid passes through the brake lines and presses against your car's brake pads. These pads squeeze the rotor, which is attached to the wheel, and the resulting friction slows your car. A warped rotor interferes with the brake system and can cause a failure to stop or damage to other parts of the system. Unfortunately, warped rotors in the front and rear produce similar symptoms. If your car is showing signs of a warped rotor, it may be best to have a mechanic determine which one is causing the problem.
Pay attention to the way your brake pedal feels when you press it. Warped rotors cause pulsating pressure in the brake lines as the pads are moved in and out with any contours that may have developed in the rotor. If you can feel a pulsation in the pedal, your rotors have likely warped. (See reference 1)
Severe warping of rotors may cause the entire frame of the car to shake. You may be able to feel this shaking not only in the brake pedal but in the steering wheel as well. (See reference 1) Shaking in the steering column may be noticeable only when you are forced to stop quickly or from high speeds.
A slightly warped rotor can cause damage without being bent enough to produce any noticeable pulsation or shaking. If you are getting significantly fewer miles out of brake pads than their manufacturer suggests, there may be a slight warp in the rotor that is causing extra wear on the pads. (See reference 2)