Short answers, pulled from the story.
The Soviet Union, officially the Union of Soviet Socialist Republics or USSR, was a transcontinental country that spanned much of Eurasia from 1922 until its dissolution in 1991. It was the world's largest country by area, the third-most populous, and a federal union of national republics governed by the Communist Party as a one-party state.
The Soviet Union formally dissolved on the 26th of December 1991, when Mikhail Gorbachev officially recognized its dissolution. A failed coup by hardline communists in August 1991 hastened the end, after which Russia, Ukraine, and Belarus seceded and all fifteen republics became independent states.
Vladimir Lenin led the new government after the October Revolution of 1917, followed by Joseph Stalin, Nikita Khrushchev, Leonid Brezhnev, and finally Mikhail Gorbachev. Boris Yeltsin, president of the Russian SFSR, oversaw the country's reconstitution into the Russian Federation.
The Soviet Union suffered around 27 million casualties in World War II, the most deaths of any country in the conflict. Germany invaded in 1941 in the largest land invasion in history, and the Soviets played a decisive role in defeating the Axis powers as part of the Allies.
The Soviet Union adopted a command economy in which production and distribution were centralized and directed by the government through Gosplan and five-year plans. Heavy industry and defence were prioritized over consumer goods, which were often scarce outside large cities, and a massive unplanned second economy grew up alongside the planned one.
The Soviet Union launched the first satellite, Sputnik 1, at the end of the 1950s, and on the 12th of April 1961 sent Yuri Gagarin aboard Vostok 1 as the first human launched into space. Its N1 rocket failed all four test launches, and the Moon landing was won by the Americans.
The Soviet Union had the largest standing military and the world's largest arsenal of nuclear weapons, peaking at over 40,000 warheads in 1986. It was the second country to develop nuclear weapons, testing RDS-1 in 1949, and also held the world's largest chemical and biological weapons programs.