Every manufactured vehicle has a set of criteria which is used to determine when you car needs servicing and at what point certain parts of the car need to be replaced this is called manufacture hand book servicing. Other commonly used terms are log book servicing and vehicle component replacement schedule.
Each vehicle has different serving requirements depending on the build and components used when the vehicle is manufactured, for example some cars require serving every 10,000 km while others every 25,000 kms.
Also at different times during the life of your car major components will need to be replaced.
The replacement of such parts is determined by either or both of the following; 1) age of the vehicle and 2) how many km the vehicle has travelled; for example the British built MGF has a component called a cambelt, the Manufactures handbook service schedule recommends the cambelt is replaced every 5 years or 100,000 which ever occurs first.
If the part is not replaced as per the schedule and fails the hole engine will need to be replaced. In the case of the MGF it's the difference between replacing one component and replacing an entire engine.
If you are like some people that have either lost or do not have the manufactures hand book don't worry because AGA automotive has a list of service schedules for all types of vehicles in our mechanical workshop.