The UK Parliament sits for a maximum of five years, after which it is dissolved and a general election is held.
Sometimes a Parliament may be dissolved much earlier than this.
On the other hand, it may run for an extended term - but this is only likely in times of war or other national crises.
When an election is called, the prime minister, who has the power to choose the election date, visits the monarch in order to dissolve the existing parliament.
This sets in motion a strict timetable, and political parties begin their election campaigns in earnest as polling day will occur 17 working days later.
The electorate, comprising of most people over the age of 18, is entitled to vote for their choice of candidate to represent their constituency and its interests in the House of Commons.
In each constituency the candidate who wins the most votes is elected to Parliament and will take a seat in the House of Commons.
The UK is currently divided into 650 constituencies.
The party that wins more than half of the seats in the Commons is said to have an "overall majority" and can therefore form the government.
If no party manages to win an overall majority then there is a "hung parliament" and some parties will combine to form a coalition government.
The second largest party forms Her Majesty's Official Opposition.