Let's start at the beginning.
Disc brakes are designed to slow or stop a vehicle's wheel from rotating. Science has proved that disc brakes are much more efficient in slowing a vehicle when compared to drum brakes. Most vehicles are now coming with disc brakes on the front and rear axles. You will only occasionally see drums on the rear axle in the 21st century.
Discs, or rotors as they are more commonly known in the US, are generally made of cast iron. In some high-end models they're ceramic or drilled and or slotted. The rotors are attached to the wheel hub. Friction material or brake pads are mounted inside the brake caliper. The caliper squeezes the brake pads against the spinning rotor when hydraulic pressure from the brake pedal is applied.
The area of contact between the pads and the rotor must be within precise specifications for the whole system to work properly.