A national insurance number is your own personal account number and is necessary for living in the UK. It acts as a reference number when communicating with the Department of Work and Pensions and HM Revenue & Customs (HMRC), and is used to record your national insurance contributions.
People born in the UK are assigned an NI number and receive a plastic number card shortly before their 16th birthday. This must be kept in a safe place.
People from abroad who wish to work in the UK may apply for a number through the Department for Work and Pensions.