The NKVD  (People's Commissariat for Internal Affairs) was a government department in the Soviet Union. It was the law enforcement agency which did the will of the All Union Communist Party. The NKVD underwent many organizational changes; between 1938 and 1939 alone, the NKVD's structure changed three times.

The public face of the NKVD was the regular police force, which acted in a similar way to other police forces. In addition to its state security and police functions, some of its departments handled other matters, such as transportation, fire guards, border guards (NKVD border troops), etc. These jobs were usually assigned to the Ministry of the Interior (MVD).

However, it is the classified (secret) activities for which it was really feared. This was the work of the Main Directorate for State Security, known as the GUGB. The NKVD also included the Soviet secret police. The GUGB protected the state security of the Soviet Union. This was done by massive political repression, including the use of sanctioned political murders and assassinations, especially in the era of Joseph Stalin, during the Great Purge.

Later publications   
After World War II, and especially after the end of the Soviet Union, there was a flood of books on the deeds of Stalin and the NKVD.

Bloch, Sidney & Reddaway, Peter. 1977. Russia's political hospitals: the abuse of psychiatry in the Soviet Union. London, Victor Gollancz. 
Conquest, Robert 2007. The Great Terror: a reassessment: 40th Anniversary Edition. Oxford University Press.
Herling, Gustav 1951. A world apart: the journal of a Gular survivor. reprint ISB	0-87795-821-1
Polyan, Pavel 2004. Against their will: a history and geography of forced migrations in the USSR.  Central European University Press, 2004. 
Rummel R.J. 1990. Lethal politics: Soviet genocide and mass murder since 1917. Transaction.  
Shalamov, Varlam Tikhonovich 1994. Kolyma tales. (transl. Glad, John) Penguin. 
Arrested as a student, Shalamov spent a total of 17 years in prison for saying, and trying to publish, things the regime did not approve of. His tales are of his experiences, and those of other prisoners.
Solzhenitsyn, Alexandr 1973–78. The Gulag Archipelago: 19181956. (three volumes). 
A history of the entire process of developing and administering a police state in the Soviet Union. It was circulated in samizdat (underground publication) form in the Soviet Union until its official publication in 1989. After the dissolution of the Soviet Union and the formation of the Russian Federation, The Gulag Archipelago became required reading in Russian high schools. The Gulag Archipelago, is both a rhyme and a metaphor used throughout the work. The word archipelago describes the system of labour camps spread across the huge Soviet Union like a vast chain of islands, known only to those who were fated to visit them.

During and after the war 
In liberated territories, the NKVD  carried out mass arrests, deportations, and executions of traitors, and other Nazi collaborators. Resistance movements included the Polish Armia Krajowa (which was neither pro-nazi or communist) , and leftover Ukrainian Nazi units (aiming to separate from the Soviet Union).

It is disputed whether or not the NKVD or Nazi Soldiers dressed as NKVD officers  were responsible for the execution of tens of thousands of Polish political prisoners in 1939–1941, including the Katyń massacre. NKVD units were also used to quell the post-Nazi partisan war in the western Ukraine (Mostly in the Lviv region), and the "Forest Brothers" in the Baltics (which lasted until the early 1950s).

References

Other websites 
 NKVD.org: Information website about the NKVD

Intelligence agencies
Soviet Union
Law enforcement agencies