For many years now disc brakes have been used on the front wheels of cars and bakkies. On some models you also find them on the rear wheels.
They replaced the drum brakes, which until the 1960s were almost universally used on both front and rear wheels.
In drum brakes the friction required to slow the wheels down comes from the linings on "shoes" being pushed outward into contact with the inside of rotating drums.
Disc brakes still use friction to slow the wheels down, but the friction now comes from "pads" clamping onto a spinning disc made of steel or cast iron. The pads are contained in a caliper straddling the disc and attached to a suspension upright. Unlike drum brakes, disc brakes have no self-servo action; their braking effort is always proportional to the hydraulic pressure applied to the pistons inside the calipers, which push the pads against the disc.
Disc brakes have several advantages over drum brakes. They are less affected by water, because any water that gets on the disc will be spun off. They can also be cooled more efficiently.
The pads consist of a tough friction lining bonded to a steel backing plate.These pads have a long service life and produce less of the clinging dust that always makes the front wheels look dirty. The friction lining on pads slowly wears down and the pads have to be replaced from time to time. Some pads incorporate a mechanism to alert the driver that it's time to replace them.
Some discs have holes to overcome problems with pads that give off gas when they get very hot.