A parliamentary system of government means that the executive branch of government has 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. There are different ways of balancing power 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. They change after their terms are over. The head of government is the prime minister, who has the real power. The head of state may be an elected president or, in the case of a constitutional monarchy, hereditary.
Examples of countries which practice parliamentary systems are most of the countries which are democracies.