During the Cold War, the Eastern Bloc (or Soviet Bloc) comprised the following Central and Eastern European countries: Bulgaria, Romania, Hungary, East Germany, Poland, Albania (until the early 1960s, see below), the Soviet Union, and Czechoslovakia. The Eastern Bloc is also often equated with the Warsaw Pact. Another organization encompassing the countries of the Eastern bloc was the Comecon.
Yugoslavia was never part of the Eastern Bloc or Warsaw Pact. Although it was a communist state, its leader, Marshall Tito, came to power through his efforts as a partisan resistance leader during World War II, and thus he was not installed by the Soviet Red Army, and he owed the Soviet leadership no allegiance. The Yugoslav government established itself as a neutral state during the Cold War, and the country was one of the founders of the Non-Aligned Movement.
Similarly, the Stalinist Albanian government also came to power independently of the Red Army as a consequence of World War II. Albania broke with the Soviet Union in the early 1960s and aligned itself instead with the People's Republic of China.
Nations within the Eastern Bloc were often held in the Soviet sphere of influence through military force. Hungary was invaded by the Red Army in 1956 after it had overthrown its pro-Soviet government; Czechoslovakia was similarly invaded in 1968 after a period of liberalization known as the Prague Spring. The latter invasion was codified in formal Soviet policy as the Brezhnev Doctrine.
The Eastern bloc came to an end with the collapse of the pro-Soviet regimes in Eastern Europe in 1989.
Today, the term former Eastern Bloc may be perceived as a politically-correct reference, instead of Eastern Europe, to countries formerly dominated by the Soviet Union.