What was the relationship between Germany and Russia during WWII?

Nazi-Germany and the Soviet Union stunned the world in 1939 by signing the Molotov-Ribbentrop pact. Nazism and communism had always been ideological enemies, yet here were the foreign ministers of the Third Reich and the USSR attaching their signatures to an agreement that limited spheres of influence between the two regimes and guaranteed mutual non-aggression.
Despite this, however, Hitler's long-term plans remained unchanged. He still wanted to invade and conquer the Soviet Union, or at least its European territories. The Molotov-Ribbentrop pact was simply a stop-gap measure to buy Hitler more time. Having secretly carved up Poland between himself and Stalin, he now felt more secure in launching his invasion of that country.
But all the time, a German invasion of Russia was an inevitability. Years before, in Mein Kampf, Hitler had maintained that Germany needed to expand; the German people needed lebensraum, or living space. This would be provided by the establishment of an empire in Eastern Europe, based largely upon land appropriated from the Soviet Union.
The Soviets were taken aback by the German invasion. In the early days of the war, the Germans advanced rapidly, conquering vast swathes of territory. It seemed to many at that time that Hitler's dream of a racial empire in the east was in the process of becoming real. Yet the Russians recovered and fought back against the German invaders. The Molotov-Ribbentrop pact was in tatters and the two dictatorships were now at war.
Such a titanic clash of ideologies guaranteed that the ensuing conflict would be an especially bitter one. Hitler called it a "war of annihilation" and that set the tone for the unimaginable horrors that were to follow. This wasn't just a war between two rival nations; it was an apocalyptic struggle between two contradictory ideologies; a fight to the death which could only involve human suffering on a massive scale. And so it proved.
Nazi ideology held that Russians were racially inferior, little better than slaves, and that's precisely how the Germans set about treating the local population after they invaded Russia in 1941. Some Nazis believed that if they treated subject nationalities such as Ukrainians and Belorussians with decency then they'd respond by allying themselves with the Germans against the Russians.
However, such voices did not prevail and ideology determined how the subject population were to be treated. As far as the Nazis were concerned, they were the lords and masters of all they surveyed. The conquered Slavs existed purely and solely to serve them. Little wonder, then, that the Germans treated the people they governed with such appalling brutality; and little wonder, too, that the Soviets exacted such ruthless revenge against their former conquerors after the war.