Short answers, pulled from the story.
Hardline members of the Communist Party launched the coup because they felt betrayed by Mikhail Gorbachev's reforms and feared losing control over Eastern European states. The New Union Treaty threatened to decentralize power among fifteen republics, prompting nationalists in Estonia, Latvia, Lithuania, and Armenia to declare independence.
The State Committee on the State of Emergency led the conspiracy with KGB Chairman Vladimir Kryuchkov, Defense Minister Dmitry Yazov, Premier Valentin Pavlov, Vice President Gennady Yanayev, Oleg Baklanov, Valery Boldin, and Oleg Shenin. Foreign Secretary Eduard Shevardnadze resigned in protest before the group formed their plan.
The coup began at 4:32 pm on August 18 when committee members cut communications to Mikhail Gorbachev's dacha in Foros, Crimea. It ended between 8:00 am and 9:00 am on August 21 when GKChP members met in the Defense Ministry and decided to send leaders back to Crimea after troops pulled out of Moscow.
Units of the Tamanskaya mechanized infantry and Kantemirovskaya armored division sent around four thousand soldiers, three hundred tanks, three hundred armored personnel carriers, and four hundred twenty trucks to Moscow. Almost no other arrests were made by the KGB during the operation.
Soviet forces suffered mass defections because soldiers refused orders to shoot civilians while Major Evdokimov declared loyalty to the Russian SFSR leadership. Boris Yeltsin climbed onto a tank to address crowds, and trolleybuses barricaded tunnels against arriving infantry fighting vehicles before Alpha Group and Vympel failed to move as planned.