The Union of Soviet Socialist Republics is, as the name would suggest, a union of republics. Each republic is generally created along the lines of ethnic groups and distinctions so as to prevent any conceptions of ethnocentrism or discrimination as well as provide some sense of ethnic equality.
Although supposedly equal in power to the central government in Moscow, the republics are in practice subservient to the Supreme Soviet and can be disbanded at will. Each republic officially retains the right to secede from the Soviet Union and has the authoritiy to regulate their particular ethnic group and that ethnic group's language and culture. Each republic also has a local Communist Party, although those Communist parties typically serve as branches of the Communist Party of the Soviet Union.
Prior to the reforms and national delimitation of 1936, the Soviet Union was made up of four republics: the Russian Soviet Federative Socialist Republic, the Transcaucasian Socialist Federative Soviet Republic, the Byelorussian Soviet Socialist Republic, and the Ukrainian Soviet Socialist Republic. Following the delimitation, the Soviet Union was remade from four republics into thirty-one republics.