The Russians took Eastern Europe in what Stalin felt was essentially compensation for the beating the Soviet Union took during World War II. Berlin was considered the ultimate prize which is why raced his armies to it during the final months of the war.
While still allied with the west, Berlin along with Germany was split up into regions under British, American, French and Russian control. When Stalin blockaded Berlin, the West i.e United States and others decided to defend it's part of Berlin and thus why the airlift occurred bringing much needed supplies into the city only along 3 routes tightly restricted and patrolled by the Soviets.
Eventually it persuaded Stalin to abandon his attempts to force the allies out of West Berlin thereby preventing him from claiming the entire city and allowing an odd western enclave surrounded by a communist nation (East Germany). It became a major point of pride for the West in the very early stages of the Cold War and led directly to the building of the Berlin Wall.
