Basically, you are just compressing some liquid against a piston which is squeezing a couple of pads against the rotor. It's just about as simple as that. Those of you with ABS have some neat stuff in between but essentially it all works the same. In order to slow down we must generate heat. This is one of the few times that we LIKE friction.
Friction is good.
The more friction, the faster we can slow down, assuming the tires have some traction of course. There are two reasons why one would like to upgrade their brakes. One, you would like to be able to stop shorter and or faster. Two, you want to get rid of this annoying sinking pedal known as brake fade. There are two kinds of brake fade.