A brake
caliper is an integral part of a
disc brake system. The caliper provides the
clamping force that presses the
brake pad into the
brake rotor. Calipers are usually cast iron or aluminum devices that slide over the brake rotor. They are typically secured by bolting to a spot on the steering knuckle.
Most calipers are operated by hydraulic means in automotive applications. They are typically fed with fluid through steel lines from a brake master cylinder located in the engine compartment. When the user presses the brake pedal, hydraulic action compresses the piston(s) in the caliper causing the clamping force.
Many cheap automotive calipers have only a single piston (often called floating), which clamps down from only one side of the rotor. Rotors for sports cars often have large dual piston calipers, or may even feature four or six piston versions for even greater clamping force.
The caliper also acts as a sink to draw heat away from the pads and rotor. During braking all that kinetic energy in your car is converted into, you guessed it, heat. The hydraulic brake fluid that fills the caliper soaks up much of the heat energy created during braking.
It is possible for the brake calipers and fluid to become overheated during excessive amounts of braking. Brake fluid is engineered to have a high boiling point for exactly this reason. Cheap brake fluids can have lower boiling points and can sometimes boil right out of the brake master cylinder if temperatures get too high.