Short answers, pulled from the story.
Operation Barbarossa began on the 22nd of June 1941 and ended on the 5th of December 1941. The invasion commenced at around 03:15 with a massive artillery barrage and air assault along a 2,900-kilometre front.
More than 3.8 million Axis troops invaded the western Soviet Union at the start. Around 10 million combatants took part in the opening phase, and over 8 million casualties had been recorded by the end of the operation on the 5th of December 1941.
Hitler renamed the operation from Operation Otto to Operation Barbarossa in December 1940, honouring the medieval Holy Roman Emperor Friedrich I. For Hitler, Barbarossa signified his belief that conquering the Soviet Union would usher in the Nazi Thousand-Year Reich. The name drew on a 19th-century German nationalist legend that Barbarossa slept in a cave in the Kyffhäuser mountains and would awaken in Germany's greatest hour of need.
Stalin believed Hitler was unlikely to open a two-front war while still fighting the British, and he feared that mobilising Soviet forces might provoke Germany. He received repeated warnings from his own intelligence services, from British intelligence, and from Soviet spy Richard Sorge, who provided the exact launch date, but he disregarded them. When German soldiers crossed the Bug River the night before the invasion to warn the Red Army, they were shot as enemy agents.
Generalplan Ost was a Nazi secret plan prepared in 1941 and confirmed in 1942 that called for the ethnic cleansing, execution, and enslavement of populations across conquered Eastern European territories. It envisaged Germanisation of the land west of the Urals over 25 to 30 years. The plan had two parts: the Kleine Planung covering actions during the war and the Große Planung covering post-war policies.
Army Group Centre stalled at Moscow's outskirts by the 5th of December 1941 due to strained supply lines, poor roads, unprepared winter equipment, and determined Soviet resistance. Germany had expected a rapid collapse similar to the fall of France but instead encountered a war of attrition. Neither Hitler nor the General Staff had prepared warm clothing or winterised tanks and artillery for a prolonged campaign.