The Gulag was the government agency that administered the penal labor camps of the Soviet Union. Gulag is the Russian acronym for "Chief Administration of Corrective Labor Camps and Colonies" of the NKVD.
Eventually, by metonymy, the usage of "Gulag" began generally denoting the entire penal labor system in the USSR, then any such penal system.
Gulag: A History, by Anne Applebaum,explains: "It was the branch of the State Security that operated the penal system of forced labour camps and associated detention and transit camps and prisons. While these camps housed criminals of all types, the Gulag system has become primarily known as a place for political prisoners and as a mechanism for repressing political opposition to the Soviet state. Though it imprisoned millions, the name became familiar in the West only with the publication of Aleksandr Solzhenitsyn's 1973 The Gulag Archipelago, which likened the scattered camps to a chain of islands."
There were at least 476 separate camps, some of them comprising hundreds, even thousands of camp units. The most infamous complexes were those at arctic or subarctic regions. Today's major industrial cities of the Russian Arctic such as Norilsk, Vorkuta, Kolyma and Magadan, were camps originally built by prisoners and run by ex-prisoners.
More than 14 million (with some authors like Solzhenitsyn estimating the total at more than 40 million) people passed through the Gulag from 1929 to 1953, with a further 6 to 7 million being deported and exiled to remote areas of the USSR. According to Soviet data, a total of 1,053,829 people died in the GULAG from 1934 to 1953, not counting those who died in labor colonies. The total population of the camps varied from 510,307 (in 1934) to 1,727,970 (in 1953).
Most Gulag inmates were not political prisoners, although the political prisoner population was always significant.
People could be imprisoned in a Gulag camp for crimes such as unexcused absences from work, petty theft, or anti-government jokes. About half of the political prisoners were sent to Gulag prison camps without trial; per official data, there were more than 2.6 million imprisonment sentences in cases investigated by the secret police, 1921-1953.
While the Gulag was radically reduced in size following Stalin’s death in 1953, forced labor camps and political prisoners continued to exist in the Soviet Union right up to the Gorbachev era. However, the camps in Siberia still house a work force of about a million prisoners.