A parliament is a legislature, especially in those countries whose system of government is based on the Westminster system modelled after that of the United Kingdom. The name is derived from the French parlement, the action of parler (to speak): a parlement is a talk, a discussion, hence a meeting (an assembly, a court) where people discuss matters.
Legislatures called parliaments typically operate under a parliamentary system of government in which the executive is constitutionally answerable to the parliament. This can be contrasted with a presidential system, on the model of the United States' congressional system, which operate under a stricter separation of powers whereby the executive does not form part of, nor is appointed by, the parliamentary or legislative body. Typically, congresses do not select or dismiss heads of governments, and governments cannot request an early dissolution as may be the case for parliaments. Some states have a semi-presidential system which combines a powerful president with an executive responsible to parliament.
The lower house is almost always the originator of legislation, and the upper house is usually the body that offers the "second look" and decides whether to veto or approve the bills.
A parliament's lower house is usually composed of at least 200 members in countries with populations of over 3 million. A notable exception is Australia, which has only 150 members in the lower house despite having a population of over 20 million.
The number of seats may exceed 400 in very large countries, especially in the case of unitary states. The upper house customarily has 20, 50, or 100 seats, almost always significantly fewer than the lower house (the British House of Lords is an exception).
A nation's prime minister ("PM") is almost always the leader of the majority party in the lower house of parliament, but only holds his or her office as long as the "confidence of the house" is maintained. If members of parliament lose faith in the leader for whatever reason, they can often call a vote of no confidence and force the PM to resign.