There have been a couple of American posters in some threads who are confused about British politics, so:
Parliament is made up of two houses, the Lords and the Commons. The Commons is the main one. Candidates from each party stand for a constituency (area) and are elected to Parliament on First Past the Post in that individual onstituency. National votes don't count.
The elected candidate becomes an MP and goes to Westminster. He may be chosen as a minister if he is skilled/ experienced or loyal enough. At present if he is a member of the Labour paty he would be in the government, if a Conservative he will be in the shadow government.
The main parties are:
Labour(Government, Leader: Gordon Brown) Centre-left
Conservatives "Tories" (Shadow Government, Leader: David Cameron) Centre-right
Liberal Democrats "Lib Dems" (Leader: Sir Menzies 'Ming' Campbell) Centre
British National Party "BNP" (Leader: Nick someone-or-other) Far-right
Green (Leader: I have no idea) Left
United kingdom Independence Party "UKIP" Euro-sceptic
Respect Multi-cultural centrist
The Prime Minister is not directly elected, but is elected as any other MP. He has the powers of the Royal Perogative, but all new legislation must be voted for by both houses of Parliament.
The House of Lords is mostly appointed "life-peers" consisting of former generals, chiefs of police, great sportsmen, businessmen, lawyers ,doctors and those who make donations to the labour party.
There are also law lords, the most senior judges in the country.
There are 92 hereditary peers, who are the remnants of the noble families, but they will be removed before too long. Collectively these are all known as Lords temporal. There are also Lord Spiritual, the most senior bishops in the Church of England.
The public occassionaly get a say in issues when referendums are held