A tune up generally includes replacement of several parts on your car. These parts may seem superficial, but failing to replace them regularly can cause decreased performance in your car, and may even lead to other problems. For example, an air filter should be replaced at least once a year; failing to replace it when it's dirty will cause your engine to get less and less of the air it needs to run properly. If the problem is left unattended, the air-fuel mixture will continue to run richer and richer —- meaning that there will be too much fuel and not enough air in the mixture —- and eventually cause other parts to fail.
As you can see, a regular tune up is important to your car's performance. A tune up should involve replacing the air filter, replacing or cleaning the spark plugs, and replacing the distributor cap and rotor. A tune up can also include replacement of the spark plug wires, fuel filter, PCV valve, and oxygensensor.
Maintenance that is not included in the basic tune up may also be required, so a yearly tune up provides a good opportunity to check the car's systems, such as the brakes and clutch; all fluid and oil levels; and the operation of any other systems that are not used or checked regularly. If the tune up is performed in spring or early summer, the air conditioning system should be checked as well, as it likely will not have been used for many months