If your car has a problem you can, if you wish, fix it yourself. That's not the case with most general aviation aircraft. For the protection of yourself, passengers, future owners of your plane, and people on the ground, the FAA requires that all repairs on certified production airplanes be done by or under the supervision of an FAA-certified mechanic. Exceptions to these rules are covered elsewhere. For now, let's look at the rules.
The rules for aircraft maintenance, repairs, alterations, and rebuilding are in CFR 14-43 (previously called FAR Part 43). It's dull reading, but tells you who can fix airplanes, what they can and cannot do, and how they keep track of their efforts. Without the help of an FAA-certified Airframe and Power-plant (A&P) mechanic, the owner is permitted to do little more than some preventive maintenance. So let's first look at what "preventive maintenance" means.
Airframe and power-plant (A&P) mechanics are licensed to work on the airframe, the engine, or both. An A&P mechanic can also perform maintenance on the entire plane.