A car is essentially a machine, and as such, it requires a certain amount of preventative maintenance in order to continue to perform. A tune up is a regularly scheduled opportunity, usually once a year, to do all of the preventative maintenance that needs to be done. Ensuring that your car gets a tune up regularly will help maintain the performance of your car and extend its life.
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. The process should involve replacing the air filter, replacing or cleaning the spark plugs, and replacing the distributor cap and rotor. It can also include replacement of the spark plug wires, fuel filter, PCV valve, and oxygen sensor.
Maintenance that is not included in the basic tune up may also be required, so a yearly check 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 procedure is done 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.
A note on newer cars: most new cars use platinum spark plugs, which do not require frequent replacement. Platinum spark plugs are often claimed to last 60,000 to 100,000 miles (95,561 to 160,934 km), or even more. These spark plugs will not need to be replaced with every tune up. Some newer cars also use an electronic ignition instead of a distributor, and therefore do not need a new distributor cap and rotor. For most cars, it is a good idea to check the owner's manual or shop manual to see what maintenance is recommended.