A pension is a way of saving money for future to provide you with an income when you retire.
Workplace pension/Company pension/occupational pension is when an employer arranges to provide their workers with a pension when they retire.
The auto-enrolment rules have made it mandatory for the employers to enrol their employees for pension on staging date of there company which means that if you're an employee, your employer will be forced to offer you a pension scheme. By 2018 all employers by law will have to contribute to their employees' pensions, but to start with only larger firms will offer this.
As an employee you can choose to say no to auto enrolment if you don’t want to join company pension. But if you do nothing, you'll be opted in.
If you don’t meet the criteria above when your employer starts enrolling workers, you will not be automatically enrolled into a workplace pension. However, you may be able to join the pension scheme if you want, if you’re not already a member. Your employer will let you know (so long as you’re 16 or over, and under 75).
It’s possible to have both a workplace pension and your own personal pension, so you could choose to continue paying into both. Or you might choose to continue with just one of them. It depends on your circumstances - for example, what you can afford and what your personal and workplace pension schemes are offering.