GULAG is an acronym of the Soviet institution that was in charge of the forced labour camps, the Glavnoe Upravlenie ispravitel’no-trudovykh Lagerei (Main Administration of Corrective Labour Camps). There were forced labour camps all around the USSR, but the largest ones were located in the most extreme regions of the country, where climatic conditions were really hard (in Siberia and the North Arctic region). These forced labour camps appeared in 1917, but their number increased considerably during Stalin's rule. After Stalin's death the number of prisoners reduced, but forced labour camps continued to exist until Gorbachev's times.
The GULAG system included all type of prisoners: real criminals (robbers, murderers,rapists...) and dissidents (those who opposed the government). But during the period of Stalinist purges, many innocent people were sent to the camps as a consequence of the imaginary state of siege and paranoia the regime experienced. Here you have an example:
In 1949 this man, called Ivan Burylov, was sentenced to 8 years in camp, because he wrote "commedy" on a ballot paper, as a protest against the farce of the elections in the USSR: although there was one only allowed party, the authorities organized elections and people felt forced to vote, because if they didn't do it, they could be accused of opposing the government. Burylov found a way of protesting, but he was caught and sent to the camp.
Ballot paper where Burylov wrote "commedy"
If you want to learn more about GULAG, you can visit an online exhibition on the following link: