A Politburo is short for Political Bureau from the German Politbüro and is the executive organization for a number of political parties, most notably for the Communist Parties. Under Marxism-Leninism the party is seen as the "vanguard of the people" and therefore usually has the power to control the state, and the non-state party officials in the politburo generally hold extreme power.
In the Soviet Union for example, the General Secretary of the Communist Party did not necessarily hold a state office like president or prime minister to effectively control the system of government. Instead party members answerable to or controlled by the party held these posts, often as honorific posts as a reward for their long years of service to the party. On other occasions, having governed as General Secretary, the party leader might assume a state office in addition. For example, Mikhail Gorbachev initially did not hold the presidency of the Soviet Union, that office being given as an honour to a former Soviet Foreign Minister.