Original: The NKVD contained the regular, public police force of Soviet Russia and the USSR (including traffic police, firefighting, border guards and archives) but is better known for the activities of the Gulag and the Main Directorate for State Security (GUGB), which eventually became the Committee for State Security (KGB).
Simplified: The NKVD is best known for the Main Directorate for State Security (GUGB), which came after the OGPU and the Cheka as the secret police agency of the Soviet Union, and which became KGB.