Short answers, pulled from the story.
The Great Purge was a political purge in the Soviet Union from 1936 to 1938, led by Joseph Stalin to remove suspected dissenters from the Communist Party. It was carried out largely by the NKVD and reached its peak between September 1936 and August 1938. Scholars estimate the death toll at 700,000 to 1.2 million.
The Great Purge is called the Yezhovshchina because its peak, between September 1936 and August 1938, came when the NKVD was led by chief Nikolai Yezhov. Stalin later oversaw Yezhov's own execution after reversing his stance on the purges in 1938.
The Moscow trials were three highly publicized show trials of former senior Communist Party leaders held between 1936 and 1938. The accused were charged with conspiring with fascist and capitalist powers to assassinate Stalin and restore capitalism, and convictions were obtained through forced confessions. The first trial in August 1936 condemned Grigory Zinoviev and Lev Kamenev, and the third in March 1938 condemned Nikolai Bukharin.
The Great Purge followed the 1934 assassination of the high-ranking official Sergei Kirov by Leonid Nikolaev. Stalin treated Kirov's death as the flashpoint to begin the purges, charging a growing group of former opponents with the murder and a widening list of offenses including treason, terrorism, and espionage.
Scholars estimate that 700,000 to 1.2 million people died during the Great Purge, including executions, deaths in detention, and deaths shortly after release from the Gulag. The Kulak Operation alone, the largest single campaign in 1937-38, involved 669,929 arrests and 376,202 executions.
The purge of the army and navy removed three of five marshals, 13 of 15 army commanders, eight of nine admirals, and 154 of 186 division commanders, including Marshal Mikhail Tukhachevsky. The case rested on German-forged documents and forced confessions. Hitler considered the Red Army less effective afterward, and historians cite the disruption in its poor performance during the German invasion.
Many victims were declared innocent, or rehabilitated, beginning in 1954. Mikhail Tukhachevsky and other generals were rehabilitated in 1957, while Nikolai Bukharin and others from the Moscow trials were not rehabilitated until 1988. Leon Trotsky was never rehabilitated by the USSR.