Mikhail Gorbachev and the collapse of Communism

Read the following link. Explain how Mikhail Gorbachev opened up the Soviet Union and paved the way for the end of Communism. Also define the words "Glasnost" and "Peristrokia".
Link 1.

Collapse of Communism

Read the four links below. Why did Communism collapse in Eastern Europe and the USSR? Why did people tear down the Berlin Wall? What happened to the Soviet Union and how did the collapse of the Soviet Union change the world?
Link 1. Link 2. Link 3. Link 4.