Why did the Blockade Occur?
The Soviet Union blockaded West Berlin in 1948 because they believed that it would force the allies out of Berlin and make Berlin entirely dependnet on the USSR. Therefore making Berlin Communist.
Stalin blocked all the supply lines, cutting off the two-million strong population of West Berlin from Western help.
The USA couldn't ram tanks into the blocks or it would cause war, so they flew to Berlin by air and supplied Berlin through a constant stream of planes bringing everything from food and clothing to oil and building materials.
Stalin couldn't shoot the planes without causing a war.
What was the Outcome?
By May 1949, Stalin realised the Western Allies were not going to give up on Berlin, so he dropped the blockade and reopened communications.