Exploitation films are a type of film that rely heavily on marketing that exploits or takes advantage of topics that are taboo, forbidden, or sensationalized and will attract a lot of popular and word-of-mouth attention without traditional marketing.
Exploitation films have existed since the earliest days of Hollywood. For Example, two films that can be viewed on my site are exploitation films from the 1930s.
"Reefer Madness(1938). This is considered THE archetypal sensationalized anti-drug movie, but it's really an exploitation film made to capitalize on the hot taboo subject of marijuana use. Like many exploitation films of the time, "Reefer Madness" tried to make a quick buck off of a forbidden subject while skirting the Motion Picture Production Code of 1930. The Code forbade the portrayal of immoral acts like drug use. (The illegal drug traffic must not be portrayed in such a way as to stimulate curiosity concerning the use of, or traffic in, such drugs; nor shall scenes be approved which show the use of illegal drugs, or their effects, in detail.)
"The film toured around the country for many years - often being re-edited and re-titled ("Tell Your Children", "Dope Addict", "Doped Youth", "Love Madness", "The Burning Question"). It was re-discovered in the early 1970s by NORML (National Organization for the Reform of Marijuana Laws) and screened again as an example of the government's demonization of marijuana. NORML may have been confused about the film's sponsorship since one of the film's distributors, Dwain Esper, testified to the Arizona Supreme Court that "Reefer Madness" was not a trashy exploitation film but was actually sponsored by the U.S. Government - a convincing lie, but a lie nonetheless."
And "Sex Madness (1938). This is another typical sex exploitation film from the 1930s - complete with wild parties, sex out of wedlock, lesbianism, etc. A chorus girl's exposure to the "casting couch" also exposes her to syphilis. Exploitation filmmakers hoped to capitalize on the taboo subjects of venereal disease, sex before marriage, lesbianism, etc. while skirting the Motion Picture Production Code of 1930 which forbade a film from containing such content. Films like this would tour the United States for years - mostly being shown in rundown, skid row theaters. This film has been re-edited and re-titled ("Human Wreckage", "They Must Be Told", "Trial Marriage", "About Trial Marriage") many times to attract the same audience to film, to take advantage of a taboo subject which may have gotten press recently or to appease local censors who disapproved of the film's content."
Exploitation films usually rely heavily on sensationalized promotion regardless of the film quality. Very often, exploitation films are low budget and low quality. But this, however, is not always true. Exploitation films sometimes attract critical attention and even cult followings.
Exploitation films often exploit events that occur in the news and in the short term public consciousness that a major film studios may not take up due to the length of time of producing a major film.
Some times anti-government films will also exploit a growing minority sentiment. For example there were many war films made about the Korean and Vietnam wars before the major studios took interest in those topics.
Some exploitation films also exploit major studio projects. The lower budget films often use a faster production schedule and can take advantage of major studio advertising on a particular topic or film. For example, Edward L. Alperson produced William Cameron Menzies' "Invaders from Mars" to beat Paramount Pictures prestigious George Pal's version of "The War of the Worlds" into the cinemas. Pal's "The Time Machine" was also beaten to the cinemas by Robert Clarke's Edgar G. Ulmer film "Beyond the Time Barrier" (1960). As a result, many major studios, producers, and stars keep their projects secret.
If you are interested in the rich, uniquely American history of exploitation films, there are two excellent books on the subject:
"Forbidden Fruit - The Golden Age of the Exploitation Film", Felicia Feaster and Bret Wood, Midnight Marquee Press, 1999.
"Bold! Daring! Shocking! True! A History of Exploitation Films, 1919 - 1959″ Eric Schaefer, Duke University Press, 1999.