When you press your brake pedal, the brake fluid applies pressure to the brakes to slow down your car. Specifically, the caliper squeezes the brake pads against the brake rotors (aka discs), which causes friction and brings your car to a stop. On each wheel, there is a cast iron brake rotor and typically two high friction brake pads that squeeze the rotor to slow it down. The brake pads wear down over time and replacing them before they go bad is critical to preserving the life of the brake rotors and calipers.
Some cars will have a small piece of metal in the brake pads that will scratch against the rotor to signify that the pad is getting too thin and needs to be replaced - this scratching will produce a loud metallic squeal when you hit the brakes. Rotors don't typically need to be replaced as often as the pads, but they can be damaged. Rotor replacement or resurfacing is commonly due to drivers not replacing the brake pads often enough. We recommend checking the thickness of your brake pads when you rotate your tires. A good rule of thumb is that you should replace your brake pads when the pad thickness is equal to or less than the metal plate it is mounted to.
Front brake pads almost always wear out faster than rear brake pads because most of the car's weight is carried in the front. Be careful not to make the mistake of only checking your rear brake pads for wear and not your front. Remember also, the heavier the car and the more stop and go driving you do, the faster your brake pads wear out.