A car tuning kit is typically specialized to cover one particular area of performance. There are some kits that may include upgrades to enhance the performance of several areas, but there is not one type of kit that is all-purpose. Tuning is usually restricted to upgrading a stock vehicle to some measure of performance for racing, even though most upgraded vehicles never see a racetrack. Car tuning covers areas like audio, interior, exterior (body), and all of the various engine systems and suspension systems. Some combination kits will upgrade the fuel delivery system, the air intake system and the ignition system, as they are all dependent on one another.
For more info, see 4 Things to Know About Car Tuning