http://www.hmrc.gov.uk/working/intro/class4.htm
Class 4 National Insurance - how much you pay
The amount of Class 4 National Insurance contributions you have to pay for any tax year is based on your profits for that year. You pay 9 per cent on annual profits between £7,755 and £41,450 (2013-14) and 2 per cent on any profit over that amount.
http://www.hmrc.gov.uk/ni/intro/benefits.htm
The contributory benefits
National Insurance contributions count towards the following state benefits:
• the basic State Pension
• the additional State Pension, sometimes called the State Second Pension
• Jobseeker's Allowance - the 'contribution-based' element
• Employment and Support Allowance - the 'contribution-based' element
• Maternity Allowance
• bereavement benefits - Bereavement Allowance, Bereavement Payment and Widowed Parent's Allowance
• Incapacity Benefit