Short answers, pulled from the story.
The Second World referred to countries aligned with the Soviet Union and its allies in the Warsaw Pact and Eastern Bloc during the Cold War. These states generally shared centrally planned economic systems, single-party governments, and medium income levels. The term stood in direct opposition to the First World, which grouped countries aligned with the United States and NATO.
Countries that were part of the Second World included the Soviet Union, Mongolia, Yugoslavia (briefly), Bulgaria, Albania, Poland, Romania, North Korea, Czechoslovakia, East Germany, Hungary, China (until 1961), Cuba, Vietnam, and others across Africa, Asia, and Latin America. The list shifted over time as alignments changed and splits occurred with Moscow.
Yugoslavia was expelled from the Cominform in 1948 following the Tito-Stalin split, ending its alignment with the Soviet Union only a few years after World War II. Yugoslavia subsequently became a founding member of the Non-Aligned Movement rather than remaining in the Soviet bloc.
The Second World as a political concept became obsolete in 1991, when the Soviet Union dissolved and many communist regimes had already collapsed during the Revolutions of 1989. The fall of communist governments in Bulgaria, Poland, Hungary, East Germany, Czechoslovakia, and Romania, combined with the Soviet Union's dissolution, ended the basis for defining a Soviet-aligned Second World.
After the Cold War, the term Second World shifted from a political alignment category to an economic one, becoming a synonym for middle-income or newly industrialized countries. Under this redefinition, countries such as Russia, India, Iran, Ukraine, Mexico, Brazil, and South Africa may be labeled Second World regardless of their historical relationship to the Soviet Union.
Sociologists have criticized the three-world theory as crude and relatively outdated. Alternative terms such as "developed", "developing", and "underdeveloped" have been proposed, but those replacements have also been criticized for reflecting a colonialist mindset. Despite ongoing criticism, the three-world theory remains widely used in contemporary literature and media.