A movie that is considered a cult classic is a movie with a fanbase that is very passionate about the film. While it can be argued that most movies have passionate fans a cult classic is usually a movie that at the time it is first released is a failure in the box office.
However due to a particular actor or subject matter later on the movie is embraced and picks up its following. Some notable cult classics are "The Evil Dead", "Clerks", "Night of the Living Dead", and "Rocky Horror Picture Show".
The movies mentioned above are huge movies and most everyone has heard of them or watched them at least one time. However when they were first released they were not as popular as they are now. "The Evil Dead" has such a hardcore following that it was turned into a musical, which can be found on youtube, and currently has a remake getting ready to be released within the next few months.