Britain is normally described as having a ´two-party system´. This is because members of just two parties normally occupy more than 85% of all of the seats in the House of Commons and one of them controls the government.
During the eighteenth century, MPs tended to divide into two camps, those who actually supported the government of the time and those who actually did not. During the nineteenth century, it became the habit that the party which did not control the government presented itself as an alternative government. This idea of an alternative government has received legal recognition. The leader of the second biggest party in Parliament recieves the title ´Leader of Her Majesty´s Opposition´, and even gets an extra salary for this role. He or she chooses a ´shadow cabinet´, thereby presenting the image of a team ready to fill the shoes of the government at a moment´s notice.
Conservative party developed from the group of MPs known as the Tories in the early nineteenth century and is still often known informally by that name. It is a party of right of centre and it stands for hierarchical authority and minimal government interference in the economy, it likes to reduce income tax and gives high priority to national defence and internal law and order.
Labour party was formed at the beginning of the twentieth century from an alliance of trade unionists and intellectuals. They are of left of centre and they stand for equality of opportunities, for the weaker people in society and more government involvement in the economy; they are more concerned to provide full social services that to keep income tax low.
Liberal Democrats were formed in the late 1980s from a union of the Liberals, who developed from the Whigs in the early nineteenth century, and from the Social Democrats. They can be regarded as centre or slightly left of centre. They put more emphasis on the environment than other parties, they believe in giving greater powers to local government and in reform of the electoral system.
There exist some smaller parties, but it is very difficult for them to challenge the dominance of the bigger ones. If any of them seem to have some good ideas, these are adopted by one of the big parties, who try to appeal to as large a section of the population as possible.
General elections are called by the monarch when the prime minister so advises. The Parliament Acts 1911 and 1949 require that a new election must be called within five years of the previous general election.