The Soviet Union was a communist country with a totalitarian regime that existed from 1917 until 1991. The official name was "The Union of Soviet Socialist Republics" (USSR). The country stretched from the Baltic and Black Seas to the Pacific Ocean. In its final years it consisted of 15 Soviet Socialist Republics (SSR's). Russia was by far the largest Republic in the Soviet Union in terms of both land area and population, and also dominated it politically and economically.
During the period of its existence, the Soviet Union was by area perhaps the largest countries, or empires, the world had ever seen. It was also one of the most diverse, with more than 100 distinct national ethnicities living within its borders. The Soviet Union was so large, in fact, that even after many associated republics gained independence, the Soviet Union's successor, Russia, remained easily the largest country by area (with Canada second), and remained quite ethnically diverse, including, e.g., minorities of Tatars, Udmurts, and many other non-Russian ethnicities.
The first leader of the Soviet Union was Vladimir Lenin, who led the Communists (then called Bolsheviks) to power in the Russian Revolution of 1917. After Lenin died in 1924, power gradually consolidated in the hands of Joseph Stalin, who led the Soviet Union until his death in 1953. Stalin was the leader from 1929 until 1953. From 1921 until 1954 3.7 mill. people were sentenced for counter-revolutional crimes. Including 0.6 mil - to death, 2.4 mill. - to prison and labor camps, 0.8 mill. - to expatriation. See Gulag.
Mikhail Gorbachev became head of the Soviet Union in 1985 and attempted to preserve the collapsing Communist regime by reducing tensions with the United States and lessening the extent of political persecution, but without abandoning the core Communist tenet of centralized bureaucratic control of the economy. These attempts failed, and the collapse of the Soviet Union occurred in 1991.
From 1945 until the collapse, The Soviet Union fought a Cold War with the USA for world domination. USSR organized its fellow communist countries in Eastern Europe ( Czechoslovakia -- German Democratic Republic -- Hungary -- Poland -- Romania -- Bulgaria -- Albania ) in the Warsaw pact to counter the perceived threat from the Western European countries, organized in the NATO pact. USSR also supported a number of pro-communist / anti-USA regimes around the world, most notably Cuba, Libya, and Syria.