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 .