Occupational or Company pensions are set up by an employer for their employees. If you're offered a company pension plan it usually makes good sense to join it, because your employer may make regular and/or lump sum payments into your retirement fund. And you will usually be able to make payments as well.
Employees who leave before retirement have various options for what happens to their pension fund, depending on their personal circumstances. When they come to retire, they'll normally receive their pension in line with the rules of the company pension plan.
An employer who doesn't run an occupational plan may offer access to a group personal pension instead. This is a collection of personal pension plans grouped together to make administration costs more effective and to maximise investment growth.