The Cheka, or secret police, was established in 1917 during the Civil War in Russia. An acronym for All-Russian Extraordinary Commission for Combating Counter-Revolution and Sabotage, it was tasked with eliminating all opposition to the Bolsheviks, including crushing anti-Soviet rebellions. In 1922, it became part of the GPU, and one year later, part of the OGPU (Unified State Political Administration), which carried out mass executions and established gulags, or forced-labor camps. The OGPU controlled both domestic and foreign intelligence and security and also carried out the collectivization process of agriculture, meant to reduce the power of wealthy peasants, known as kulaks.

In 1934, the OGPU became the NKVD, which conducted a deadly purge of high-ranking Soviets who were thought to be opposed to Stalin. The NKVD went through various reorganizations. From 1954 until the dissolution of the Soviet Union in 1991, the intelligence organization was known as the KGB, which stands for the Committee for State Secrecy. The KGB conducted both foreign intelligence operations and crackdowns on dissidence within the Soviet Union.

The NKVD was a larger organization that had many functions.  The Cheka and the KGB, were both smaller organizations with fewer functions.  The Cheka was actually part of the NKVD.  The KGB was something of a successor to the NKVD.  It was one of two organizations that was created when the NKVD was disbanded in 1954.

The Cheka was just a secret political police organization.  It was part of the NKVD, which was a much bigger organization.  The NKVD also did regular police work as well as things like espionage, assassinations, keeping up the system of prisons known as the Gulag.

After the NKVD was broken up, the KGB took up some of its functions.  The KGB became the secret police for internal affairs but was also responsible for counter-intelligence within the Soviet Union and for espionage outside the country.