Short answers, pulled from the story.
The Revolutions of 1989 were driven by a combination of Soviet economic decline, Mikhail Gorbachev's reform policies of glasnost and perestroika introduced in 1986, and mass popular uprisings across Eastern Europe. The Chernobyl disaster in April 1986 accelerated the case for openness, and Gorbachev's renunciation of the Brezhnev Doctrine in July 1989 removed the threat of Soviet military intervention against reform movements.
Poland was the first Warsaw Pact country to break free of Soviet domination. On the 4th of June 1989, Solidarity won an overwhelming victory in Poland's first partly free elections, capturing all contested seats in the Sejm and 99 out of 100 Senate seats. Tadeusz Mazowiecki became Poland's first non-communist Prime Minister on the 24th of August 1989.
The Berlin Wall fell on the 9th of November 1989. The opening came after regime spokesman Günter Schabowski announced in a live TV press conference that new border rules were in effect "immediately, without delay." Border guards, overwhelmed by crowds and receiving no orders to use force, opened the gates. Sections of the wall were torn down by citizens with hammers and chisels.
Romania had the bloodiest revolution of 1989. Over 1,000 people died, including more than 100 children; the youngest victim was one month old. Nicolae Ceaușescu and his wife Elena were executed by firing squad on Christmas Day 1989 following a hasty trial broadcast on Romanian television.
The Pan-European Picnic was a border event held near Sopron, Hungary in August 1989, organized with the involvement of Otto von Habsburg and Hungarian Minister of State Imre Pozsgay. Thousands of brochures were distributed among East German holidaymakers in Hungary inviting them to an event near the Austrian border. Thousands of East Germans crossed into Austria, triggering the largest escape movement from East Germany since the Berlin Wall was built in 1961, and accelerating the collapse of the East German state.
The Soviet Union was officially dissolved on the 26th of December 1991, when the Supreme Soviet ratified the Belavezha Accords and voted itself out of existence. Mikhail Gorbachev had resigned as Soviet president the previous day, the 25th of December 1991. The Accords themselves had been signed on the 8th of December by Yeltsin and his counterparts from Ukraine and Belarus.