How the notion of "Eastern Europe" was born

Geographically speaking, eastern Europe incorporates a handful of countries at the border of Europe, ending with Russia and the Caucasus countries. Politically speaking, eastern Europe includes all eastern and central European countries, which were under Soviet control for 5 decades since the end of World War II.

In 1919, the Bolsheviks took power and communism was installed in what became the USSR. Though communist parties spread all throughout Europe trying to spread their ideology, other Soviet attempts were short-lived (example Soviet Hungary or Soviet Bavaria). In 1939, Hitler who was forseeing a new war, signed a secret pact with the USSR, through which Russia was free to invade territories of Romania, Poland and Baltic states without meeting any opposition from Germany and viceversa. The war began, but not long after Hitler and Stalin came face to face on the eastern Front in one of the greatest battle of modern times.
In 1945, Hitler was defeated. America's Pacific war was not yet over, but following further Soviet Union's military involvement, a final victory ws obtained. Roosevelt and Churchill agreed to hand over to Stalin control over half of Europe. The Yalta agreement was signed on 11 February 1945 in Crimea.

Churchill, Roosevelt and Stalin in Crimea, during the Yalta conference

Post-Yalta agreement, communist regimes were banned in western Europe and North American, where they had spread post World War I..
Despite rejecting the idea of communism, the countries of eastern and central Europe found themselves forced to accept it: Soviet Gulags were filled with millions of people (intellectuals, communist opponents etc) and the economies were seized.
In 1989, the Soviet Union collapsed.. However, the notion of East and West remains strongly ingrained in people's conscience.