USSR stands for "Union of Soviet Socialist Republics." The USSR was a socialist state that existed from 1922 to 1991, consisting of Russia and many surrounding countries. At its peak, the USSR covered over 8 million square miles, according to About.com.
The USSR was founded in 1922 after the Russian Revolution of 1917 overthrew the czar and the Russian monarchy. During its existence, the USSR was by far the world's largest country, and also one of the most diverse, with over 100 different nationalities contained within its boundaries. Present-day countries that comprised the USSR at the time include Armenia, Azerbaijan, Belarus, Estonia, Georgia, Kazakhstan, Kyrgyzstan, Latvia, Lithuania, Moldova, Russia, Tajikistan, Turkmenistan, Ukraine and Uzbekistan.