The House Committee on Un-American Activities, "HCUA" (the name was switched around to make it imply from the view of a leftist that the committee itself was unamerican) became the "HUAC" which grew from a special investigating committee established in 1938. At that time, its work was aimed mostly at German-American involvement in Nazi activity. It became a standing (permanent) committee in 1945. In 1969, the House changed the committee's name to the Committee on Internal Security. The House abolished the committee in 1975 and its functions were transferred to the House Judiciary Committee.
Little of note came from its investigations of Nazis, but the committee came into its own when it acted on suspicions that some people with Communist sympathies and links worked for the U.S. government. The background to this was the fact that radical students in the 1930s had often been attracted to Marxism, particularly in the "Popular Front" era. These people had reached positions of power by the late 1940s. Conservative voices in Congress tended to be extremely suspicious of such people, believing that these Communists had dual loyalty, and were either legal or ideological agents of the Soviet Union and Joseph Stalin. There were fears that such agents were actively working to overthrow the United States from within, and thus had to be forcibly removed from any positions of influence. In particular, the committee, with the leadership of Martin Dies and Richard Nixon, brought about the trial and imprisonment of Alger Hiss.
The committee eventually earned a reputation for being somewhat paranoid and overzealous, and soon even some anti-Communists were not comfortable with the idea of "un-American activities".
Later the committee looked into alleged Communist propaganda by Hollywood. This led to the blacklisting of a number of leftist scriptwriters known as the "Hollywood ten" after such, subsequently largely discredited, accusations were made against them. Charlie Chaplin was another target. This resulted in his lifelong exile in Switzerland.
Certainly very little propaganda made it into their films. Only one film, Mission to Moscow, was ever found to have any traces of such influence, and it was produced as much out of enthusiasm for the Soviet Union's role as an ally in World War II as out of Communist influence.
By 1951, a similar entity established in the Senate was largely under the control of Republican Senator Joseph McCarthy. To many at the time HUAC's investigations seemed to have lost their original aim, and had taken on the aspect of a McCarthyist "witch hunt", due to the Senator's demands that witnesses "name names" of those sympathetic to the Communists. Any value that HUAC had added disappeared in the turmoil of McCarthyism.
HUAC's last gasp was an investigation into the New Left, but times had changed. Targets like Jerry Rubin and Abbie Hoffman had much less to lose than the screenwriters and teachers of the early purges, and were able to monkey-wrench the theater of the show-trials, turning them into a kind of Dada or happening. In this manner the radicals increased their own credibility with their young followers, while making the old men behind the desks look foolish.