Operation Barbarossa was the codename for Nazi Germany's invasion of the Soviet Union during World War II that commenced on June 22, 1941. The operation was named after the Emperor Frederick Barbarossa of the Holy Roman Empire, a leader of the Crusades in the 12th century; according to Nazi ideology the conquest of 'inferior races' was similarly righteous. It is not to be confused with the war on the Eastern Front in its entirety. Operation Barbarossa lasted from June 1941 to December 1941, while the war on the Eastern Front lasted from June 1941 to May 1945 when the Germans surrendered.
The original German goal was the rapid conquest of the European part of the Soviet Union, west of a line connecting the cities of Arkhangelsk and Astrakhan, often referred to as the A-A line . The failure of Operation Barbarossa arguably resulted in the eventual defeat of Nazi Germany, and was a turning point for the fortunes of Adolf Hitler's Third Reich. Operation Barbarossa opened up the Eastern Front, which ultimately became one of the biggest theatres of war in human history alongside the Second Sino-Japanese War, with some of the largest and most brutal battles, deadliest atrocities, terrible loss of life and miserable conditions for Soviets and Germans alike.
Quoted from Wikipedia's page on Operation Barbarossa
At 5:30 a.m. on 22 June 1941, the German ambassador met with Molotov to announce a declaration of war on the basis of gross and repeated violations of the Russo-German Pact. The two largest and most powerful armies ever assembled confronted each other along a 3,000 kilometer line from the Barents Sea to the Black Sea. While the Russians were well aware of German preparations, and were tipped off to the impending invasion by both their own intelligence, as well foreign sources, the Germans achieved total surprise. The Germans employed three army groups (North commanded by Field Marshal Wilhelm von Leeb, Center commanded by Field Marshal Fedor von Bock, and South commanded by Field Marshal Gerd von Rundstedt), and planned to destroy all Soviet resistance in swift advances on Leningrad, Moscow, and Kiev. Hitler threw 183 divisions into the assault, while the Nazis faced 170 divisions, which represented 54 percent of the Red Army's total strength. Subsequently, the German armies were to occupy a line reaching from Archangel on the White Sea to Astrakhan on the Caspian Sea.
By the end of July the Germans controlled an area of the Soviet territory more than twice the size of France.