Stalin felt powerless because of the Marshall plan, so he wanted to stamp his authority on Berlin by blocking everything. He thought that he would force the Allies out of Berlin and make Berlin entirely dependent on the USSR
Stalin blocked all the supply lines (vital roads, railways, canals, etc) going into West Berlin in order to cut off any help and starve out the people of West Berlin in order to take over all of Berlin. If the USA would try to ram the road blocks or railway blocks it would be seen as an act of war.
The USA did not want to lose West Berlin because they feared that West Germany would be next. They overcame the shortages by accessing Berlin through the air. Every 3 minutes for 10 months an aeroplane would fly over Western Berlin to drop off supplies (food, clothing, coal, oil, etc: roughly 200,000 flights total). USSR did not shoot the aeroplanes from the USA.
By 1949 it was clear that the Berlin Blockade would not make the Western Allies give up Berlin, so Stalin reopened communications.
The Soviets build a wall between East Berlin and West Berlin to stop East Berlin-ers from being influenced by the West Berlin lifestyle