The leader of the executive Cabinet, the Prime Minister, is usually the head of government - at least in practice. In most parliamentary systems the Prime Minister and the members of Cabinet are also members of the legislature. The leader of the leading party in the Parliament is often appointed to Prime Minister.
Under the parliamentary system the roles of head of state and head of government are more or less separated. In most parliamentary systems, the head of state is generally a ceremonial position, often a monarch or president, however sometimes retaining duties without much political relevance, such as Civil Service appointments. In many (but not all) parliamentary systems, the head of state may have reserve powers which are usable in a crisis. In most cases however, such powers are either by convention or by constitutional rule only exercised upon the advice and approval of the head of government.
Parliamentary systems vary as to the degree to which they have a formal written constitution and the degree to which that constitution describes the day to day working of the government. They also vary as to the number of parties within the system and the dynamics between to the parties. Also relations between the central government and local governments vary.
A Westminster System is a particular type of parliamentary system in which the head of state has considerable reserve powers which are in practice limited strongly by convention rather than explicit constitutional rule.
The term can also be used for governance at other than central government level. An example is the city of Oslo, which has an executive council as part of a parliamentary system.