If you take a modern disc brake apart, you’ll find it’s made up of three main parts. There’s the rotor (or disc); the big, round shiny thing that looks like a metal boater hat. Then there’s the caliper which is the part that wraps over the edge of the rotor and when you press the brake pedal, grabs the rotor like a fist closing over it. Inside the caliper is the third part. Actually, it’s usually two parts; the brake pads, and one lives inside each half of the caliper, pressing against both sides of the rotor. These are the actual bits that create the friction that slows the rotor down (and therefore the wheel and tyre) when you need to stop.
Why are these separate from the caliper when they work in unison? Because the brake pad is the consumable, wearing part; the bit that can be thrown away when it’s worn out (by the friction inherent in the way car brakes work) and replaced cheaply (because you don’t have to replace the entire caliper).
You may remember in the old days when your shoes wore through at the bottom, they weren’t thrown away. Instead you took them to a cobbler and he or she re-soled them. Replacing brake pads is like re-soling shoes; it gives the same old shoe a new lease on life. The trick is to keep an eye on how worn they are so you can replace them before they wear out completely and stop working.
Now find out about how ABS works.