A parliament is a national or regional legislature, typically used in European-style governments where there is no separation of powers between the legislative and executive branches of government. In this "parliamentary style" government, the Prime Minister is selected by the political party or a coalition of political parties that has a majority in the parliament.
In England and among those who follow English politics, the term "Parliament" (without the definite article, and with a capital "P") is frequently used to denote the legislature of the United Kingdom, which is composed of the Queen, the House of Lords and the House of Commons.
The "Parl-" root means "to speak," as in parley.
The Members of Parliament (MPs) usually represent a political party. Each party is formed by people with broadly similar political views. In most cases, MPs are elected by the population according to one of a number of electoral systems. Some parliaments use a "first-past-the-post" system where each MP represents a constituency and is the candidate who received the most votes from residents of the constituency. Other parliaments use a proportional representation system, where the number of MPs from each political party is proportional to the total votes for that party across the whole electorate. There are many other varieties of electoral systems, most of which are variations on these two themes. In rare cases (for example the British House of Lords), the members are not elected by the population but are appointed in some other way.
Many parliaments are made up of two different houses - for example Australia's House of Representatives and Senate. The electoral systems of the two houses are usually different meaning the mix of MPs from the various parties will also generally differ.
Usually in a parliamentary system, the government is formed by MPs from the party which has the majority of members. The leader of that party is the head of government, often called the Prime Minister. As the government party has a majority of members, it can usually expect to win any votes in the parliament. In the situation where no one party has a majority, there are two possibilities. The first is a minority government with members of one party only, which must rely on the support from members of other parties to win votes. The other possibility is a formal arrangement between two or more parties to form a coalition government with a majority of MPs, where the government members are from all coalition partners.
This contrasts with presidential systems, where the members of the government (in particular the President) are not members of the legislature.
The Prime Minister is not elected for a fixed term, but serves for as long as he has the "confidence" of Parliament -- that is, as long as he enjoys their support. Usually, the Prime Minister (and the rest of the government) is removed after an election at which his party failed to retain a majority in parliament. Occasionally, most often during a period of minority government, the Prime Minister is removed upon losing a vote of no confidence, or resigns after losing a major vote on an issue they have championed (as Italian Prime Minister Romano Prodi did in February 2007). In other cases members of the government party may choose to change their leader in which case the new leader becomes Prime Minister, as happened in to Margaret Thatcher in Britain in 1990.
First-past-the-post elections are more likely to produce a parliament with two major parties, making minority government unlikely. Proportional voting systems allow third parties a chance to win some representation if they can win a small amount of the national vote without winning a majority of any districts. They are more likely to result in a minority or coalition government. In order to make it easier to form a majority government, proportional voting systems often favor large parties for example through thresholds or the way the number of seats are calculated from the votes.
Rules of order
Parliaments and other governing bodies have "rules of order" which govern who may speak, and how votes are taken and counted. These can be very complicated.
To see why these rules are needed, consider this possibility. Suppose a vote is taken when only 300 of the 435 members are there. Suppose 200 people vote for the motion and 100 people vote against it. Should the motion pass? A majority of those who were present voted for it, but the 200 people who voted for it are less than half of the entire membership. There's no way to tell how the 135 absent members would have voted; if all of them had voted against the motion, it would have been defeated.
In this case, the rule that applies is called the quorum rule, and it usually says that as long as there are sufficient members present, votes can be taken and the majority of those present prevails.
"Rules of order" are often called parliamentary procedure. Many organizations adopt rules of order which are broadly similar to each other. A popular reference book called "Robert's Rules of Order" gives a detailed outline of parliamentary procedure. In a small organization, like the governing body of a club or a town government, many members may not be completely familiar with all the rules. There may be a member called a "parliamentarian" whose job is to know the rules and act as a sort of referee for enforcing them.
Whig historians traced the British Parliament's lineage back to the Anglo-Saxon Witengemot, an advisory council to the king which also had the right to choose his successor. However, the first modern British Parliament dated from Edward I, who summoned nobles and churchmen, but also two representatives from each shire, city, and town, to approve a new tax.. Over the years, this was Parliament's main power: if it disapproved of what the king did, it would refuse to vote him taxes. Moreover, it would vote taxes for only limited times so that he would have to re-summon Parliament.
Effects on United States Government
Most historians agree that the bicameral nature of the United States Congress was derived from the principles of Parliament, albeit with the rights of Lords removed.