The USSR stands for Union of Soviet Socialist Republics. It was also called the Soviet Union. It was a communist country in northern Eurasia, which existed since a bunch of Russians revolted against their Tsar. Later a bunch of East Germans and another bunch of random people, who were living in the good side of Berlin at the time, destroyed a wall which had something to do with "A divided country". That set the USSR and its empire into decline.
The USSR is infamous for being in a war with the USA, in an act called "The Cold War". Anyways, they never even shot each other, they just hated their methods of Capitalism vs Communism. Capitalism won just because the Soviets screwed up communism (and their own people).
Aaah...The USSR. Such a wonderful place to be it was. They paid anything you needed, they gave you free food, education, and anything you wanted. At least they pretend they gave you everything you wanted. Good things were in such short supply you had to queue for hours to get basic stuff you needed. There was one thing they certainly didn’t give you, the right to own your own goddamn stuff.
No, it was Marxist. The USSR wasn't actually "communist" par say, because in the Marxist scheme of things a socialist society must always come first before a communist society. At least in the beginning, the USSR was socialist and they really meant to become a communist society later. Just they never managed a free, egalitarian communist society, they never got further than a horrible Dictatorship
From Humble Beginnings
The Soviet Union started a night when the formerly mentioned Russians on steroids decided to kill everyone on a palace and begin their own republic, free from elitism, instead of having the Tsar's (yeah, "Tsar" actually means "Cesar", or "emperor") empire. This happened mostly because their current Tsar, Nicholas II, was incompetent, and because it was fancy at the time to be a republic. Since they got pwned in World War I, they decided that it was the Tsar's fault, and that he didn't deserved to rule the empire. Then somebody shouted "Hey! I heard that being a Republic was awesome!", and everyone followed him. His name was Lenin.
Lenin and his buddies, the Bolsheviks, decided it was time for some reforms. So, during the October Revolution, ol' Lennie and his pals stormed the Winter Palace at Petrograd (St. Petersburg) and entered out of World War I at the Treaty of Brest-Litovsk. The Bolsheviks continued the Civil war with the whites, or, monarchists, and came out on top in 1922. The tsar and his family were executed. The Bolsheviks still had to deal with other wars, but they were external, with Russia's neighbors such as Poland, Finland, Lithuania, Latvia, and Estonia.
Then it was all fun and games, they even survived the great depression, 'til ol' Vladdy died. So a cold hearted bastard with a mustache (Be stated, his moustache wasn't Hitler's), named Joseph Stalin got to power. Then he allied with the aforementioned Hitler in World War II for a while, just because they were useful to each other since Hitler hated Communists (that stupid conservative Nazi...I wouldn't blame him for hating communists though). Then, ol' Adi (Adolf) betrayed ol' Stalie and then the USSR helped the USA win the war. Adi got pwned and went to a bunker to kill himself with a gun.
After the war
In the aftermath of the war, with American nukes used on the Japanese, the USSR started making their own. The USA then believed the USSR was going to use those nukes on them, and decided the communists were evil, which ended up with USA hating the USSR. During this period the USA amd the USSR were considered very "cold" between each other, and that's why they called it the cold war (Not because they didn't attack each other, that's plain stupid, what has cold having anything to do with not using rifles...). During this time, the USSR started spreading communism over countries like North Korea and Vietnam. Cuba had complex relations with the USSR.
The end of the USSR
Then, somewhere between the Challenger space shuttle exploding and Nirvana becoming the greatest band in the world, a symbol called The Berlin Wall, which was something like a wall dividing both sides, and all that ethics stuff which I won't talk about since its boring, was teared down by East Berliners on steroids who hated their lives. With the wall's fall, the USSR died, and fake Democracy triumphed.
The USSR turned into 15 independent states: Armenia, Azerbaijan, Belarus, Estonia, Georgia, Kazakhstan, Kyrgyzstan, Latvia, Lithuania, Moldova, Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan.
The Soviet Union loved military parades. Particularly on October Revolution Day and May Victory Day, the Soviet military would pull out all the stops and stage massive parades of military equipment and personnel in Red Square, often passing in review before aged Soviet leaders who looked as if they were propped up.