Members of Parliament fulfil two key roles in the administration of the country - either helping to shape government policy by becoming a minister, or scrutinising the work of the government from the backbenches or as part of a select committee.
Technically, all members of both the House of Commons and the House of Lords are members of Parliament.
However, convention dictates that the term member of Parliament, or MP, only applies to members of the House of Commons.
According to the UK constitution, all government ministers must also be members of Parliament - including the prime minister.
In recent years, secretaries of state, who are given responsibility for an entire government department, have tended to hail from the House of Commons - where their work can be scrutinised by elected MPs.
Government frontbenches in the Lords, meanwhile, have been populated by more junior ministers.
But the appointment of Peter Mandelson to the upper chamber and as secretary of state for business provoked controversy - partly because it bucked this trend.
The precise number of members of the House of Commons varies from time to time, depending on changes to the electoral boundaries, but it is currently 646.
The number and size of constituencies is kept under constant review by the various Boundary Commissions in association with the Electoral Commission.