You will pay tax if a company car is made available to you for your private use (this includes commuting) or you are provided with free or subsidised fuel for private use in that car. The tax you pay is broadly determined by three factors:
In general, the benefit charge is lower for cleaner and cheaper cars. This is to encourage you and your employer to choose cars which are less damaging to the environment.
This page provides links to information to help you understand how the benefit charge is calculated.