The Soviet Union of Soviet Socialist Republics (USSR) is a former country which included the modern-day Russian Federation and a number of adjacent states, including the Baltic states (Latvia, Estonia, Lithuania), Ukraine, Kazakhstan, and others. The USSR was a major world power during its existence, which began with the Russian Revolution of 1917 and continued until its collapse in 1991. The Soviet Union and the United States were major rivals from the end of WWII until the late 1980s, creating a conflict known as the "Cold War" where the superpowers fought each other through proxies. Many people believe that billions of people could have been killed if nuclear war broke out between the two countries, but thankfully it never did.
The Soviet Union was founded in 1917, when the Russian peasantry got fed up with the monarchy, presided over by the uncharismatic Czar Nicholas II. Some intellectuals in Russia embraced the idea of Communism, a socialist philosophy promoted by philosopher Karl Marx in his 1848 political manuscript, The Communist Manifesto. The basic idea of communism was to institute a classless, stateless society where the means of production were owned by the people. Through revolutionary activity, these intellectuals eventually got their way, toppling the Czar and executing the entire royal family. Although it sounded like an okay idea on paper, unfortunately the implementation of communism in the USSR was profoundly corrupt and deadly to many.
The Soviet Union was founded around by a revolution led by the Bolshevik party, under the leadership of Vladimir Lenin. After five years of unrest and purges between 1917 and 1922, it finally unified, and was recognized by other nations around the world. The country was ruled by only one party, the Communist Party. When Lenin died in 1924, he was replaced by Josef Stalin, who later became known as one of the most brutal mass murderers in history.
The Soviet Union under Stalin was a terrible time, characterized by artificial famines and the incarceration of millions of people in forced labor camps called gulags. The actions of Stalin are estimated to have led to the deaths of more than 60 million people. As WWII hit in the late 1930s, the USSR became an ally of the United States, France, and the United Kingdom in the war against the Axis Powers (Germany, Italy, Japan). After a lengthy campaign by the Nazis in an attempt to invade the country, Hitler's forces were eventually beaten back, with most of the invaders dying.
After WWII, the Soviet Union consolidated its influence in Eastern Europe, an area that became known as "behind the Iron Curtain," partially because of the political repression and media blackout there. The West aligned itself against the Soviet Union, starting the lengthy Cold War. On a more positive note, this also ignited the Space Race, where the USA and the USSR competed to see who could achieve more space "firsts" than the other. The USSR succeeded in launching the first satellite and the first man into space, while the USA succeeded in getting the first man on the Moon.
The Soviet Union collapsed in 1991 after its citizens decided they preferred capitalism. This caused Moscow to relinquish several satellite states it had previously been controlling, resulting in a multiplicity of new nations being created. Today, relations between the Russian Federation and the West are somewhat strained, but better than they were in the Cold War days.