The Prime Minister is the leader of the political party which has the most MPs and forms the Government.
So David Cameron is the Prime Minister of the UK because he is the leader of the Conservative Party, which currently has the most MPs in the House of Commons.
The Prime Minister leads the Government and has final responsibility for all policy and decisions.
He or she is helped and advised by their Cabinet, which is a group of senior ministers, many of whom are in charge of government departments like the Department of Health or the Department for Education.
Unlike the USA, where citizens vote for a president, we in the UK do not vote for our Prime Minister. In general elections we vote for who we want to be our MP and represent our constituency.
The Prime Minister appoints members of the Government and oversees the Civil Service.