Explanation -National Insurance

You pay National Insurance contributions to build up your entitlement to certain state benefits, including the State Pension.

You pay National Insurance if you’re:

16 or over
an employee earning above £149 a week
self employed making a profit over £5,725 a year (unless you get an exception)

The exact amount you pay depends on:

how much you earn
whether you’re employed or self-employed

You may also want to pay voluntary contributions to make up for gaps in your National Insurance record. For example, you can have a gap when you weren’t working and didn’t get any state benefits.
When you stop paying

If you’re employed, you stop paying Class 1 National Insurance when you reach the State Pension age.

If you’re self-employed you stop paying:

Class 2 National Insurance when you reach State Pension age (or up to 4 months after this to pay off any contributions you owe)Class 4 National Insurance from the start of the tax year after the one in which you reach State Pension age

Further information can be found at http://www.hmrc.gov.uk/ni/