Most vehicles have two sides to their braking system. The mechanical side (pads, rotors, calipers) and a hydraulic side (master cylinder, lines and fluid). The mechanical side is what most people are familiar with. Usually located inside each wheel is a rotor, a caliper and the pads. The purpose of the these components is to slow the vehicle by applying pressure on the pads to the spinning rotors. The calipers carry the pads and apply the force to the pads. These components are known to wear out over time due to constant use. Visual inspection is the best way to make sure your brakes are wearing correctly and are at a safe thickness. Wear is determined by many factors, weight of the vehicle, frequency of stops, types of use and the overall design.
While your brake pads wear the most, the rotors can also be worn. Almost all brake rotor manufacturers have a minimum thickness spec. Once the rotor has reached this point, it needs to be replaced. It is also a good idea to just replace the rotors, having rotors "turned" simply cuts them down to a smooth surface. It does not replenish the worn material.
The hydraulic side is the fluid control side. When you push on the brake pedal, you are pushing a special fluid through a series of pipes to the calipers to then push the pads against the rotors. This fluid is designed to withstand the heat that is generated from the brakes when they are used. Since fluid can not be compressed, it is ideally suited for a braking system. The one main draw back is that most brake fluid is hygroscopic. Meaning it attracts water. When more and more water contaminates the brake fluid over time, it reduces the brake fluid's ability to retain heat by lowering the boiling point. When fluid boils, it turns to a gas and can now be compressed. When you boil your brake fluid, your brake pedal can go very soft and dramatically reduce braking function. Flushing your brake fluid about every two years is a good practice. Unless you hit the track, brake fluid should be no more than 180 days old if DOT 4 fluid is being used. Some racers put in higher temp fluid that can handle even more than your standard street DOT 4 brake fluid.
Here is a great local source for brake fluid: https://www.ogracing.com/car/brakes?cat=90