Some people have their car serviced religiously every year. A full service can cost upwards of around two hundred pounds and often is not needed every year. Many manufacturers and makes and models of cars have different service schedules. Sometimes you may only need a minor service where as other times a major service may be required. Often you can find the service schedule in the owners handbook or by looking it up on the manufacturers website. All garages that carry out services should be able to tell you what components will need changing when. For example, on some services on petrol cars, you may need to have the spark plugs replaced, but this does not need to be done every year.
If you have hardly done any miles since your last service but it has been over a year, then you may decide to just go for an interim service. This usually includes an oil and oil filter change, air filter change and a visual inspection of many parts of the vehicle. Often it is a good idea to have this done just prior to an MOT so you have an idea of what may need to be replaced when it comes to the time.