The Speaker is the formal title of the Chairman or Chairwoman of the House of Commons. By extension, this title is also used for the person running the meetings of other institutions, such as the House of Representatives in the United States.
The Speaker of the House of Commons is always a Member of Parliament, and is elected by the other members of the House. By convention, he or she does not join in the debates of the House. He is seen to be a neutral Chairman. (Early Speakers - the first, Sir Thomas de Hungerford, is recorded in 1376 - had the job of reporting the deliberations of the House to the King. This was a position of honour - but not always a safe one.)