If you make a car available to your employees and it is available for private use (including commuting), your employees have to pay tax on the benefit of this. In the same way, they have to pay tax if you provide free or subsidised fuel for private use in that car.
Where these benefits are taxable, employers and other providers (but not employees) are liable to pay Class 1A NICs.
In general, the benefit charge is lower for cleaner and cheaper cars. This is to encourage you and your employees 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.