Brake judder is usually perceived by the driver as minor to severe vibrations transferred through the chassis during braking.

The judder phenomenon can be classified into two distinct subgroups: hot (or thermal), or cold judder.

Hot judder is usually produced as a result of longer, more moderate braking from high speed where the vehicle does not come to a complete stop. It commonly occurs when a motorist decelerates from speeds of around 120 km/h (74.6 mph) to about 60 km/h (37.3 mph), which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions, or hot spots. Hot spots are classified as concentrated thermal regions that alternate between both sides of a disc that distort it in such a way that produces a sinusoidal waviness around its edges. Once the brake pads (friction material/brake lining) come in contact with the sinusoidal surface during braking, severe vibrations are induced, and can produce hazardous conditions for the person driving the vehicle.

Cold judder, on the other hand, is the result of uneven disc wear patterns or disc thickness variation (DTV). These variations in the disc surface are usually the result of extensive vehicle road usage. DTV is usually attributed to the following causes: waviness and roughness of disc surface,misalignment of axis (runout), elastic deflection, wear and friction material transfers.Either type could potentially fixed by ensuring a clean mounting surface on either side of the brake disc between the wheel hub and brake disc hub before usage and paying attention to imprinting after extended usage by leaving the brake pedal heavily depressed at the end of heavy usage. Sometimes a bed in procedure can clean and minimize DTV and lay a new even transfer layer between the pad and brake disc. However it will not eliminate hot spots or excessive run out.

Extracted from Wikipedia