A National Insurance Number (NI Number) is the UK tax authorities means of recording your income tax payments and National Insurance Contributions (NICs). NICs are calculated as a percentage of your gross pay. There are fixed earnings thresholds and limits that are used to calculate liability.
Your NI Number is unique to you and will never change.
An overview and further details of the National Insurance scheme is on the Government's website.
Details of how to obtain an NI number can be found on the University's Payroll website.
Income tax is a levy imposed by Government on the earnings of UK workers. It is known as Pay As You Earn (PAYE) and is applied as a percentage of 'taxable income'. Different rates apply to different levels of taxable income - the higher your income, the higher the percentage of tax is paid.
Income tax is adminstered by HM Revenue and Customs. Further details can be found on their website http://www.hmrc.gov.uk.