A parliamentary system of government means that the executive branch of government must have the direct or indirect support of the parliament. This support is usually shown by a vote of confidence. The relationship between the executive and the legislature in a parliamentary system is called responsible government.
The separation of powers between the executive and law making branches, is not as obvious as it is in a presidential system, so there are different ways of balancing powers between the three branches which govern the country (the executive (or ministers), the law makers and the judges).
Parliamentary systems usually have a head of government and a head of state. The head of government is the prime minister, and the head of state often being an elected (either popularly or through parliament) president or hereditary monarch.