A classic film is one whose popularity has continued beyond its initial release. A classic film is still relevant long after its newness has worn off, and a true classic will never feel dated or clich'd. The most classic of classic films are those films that essentially everyone has seen-or at least has heard of-and appreciates. Classic films are an undeniable part of popular culture. Classics can come from any genre and any era, although it usually takes some time for the longevity of a film's reputation to be beyond question.
There is not a single characteristic that determines what will make a film into a classic. Some classics are artistic masterpieces that are fine examples of the filmmaker's craft (Citizen Kane), and others become classics because of the sheer scale of their popularity (Gone with the Wind). Some classics are remembered for their place in film history, because of the way that they established genres or broke with traditions (Psycho, 2001: A Space Odyssey).
It's more difficult to determine whether more recent films will become true classics, but many films made in the past 20 years are so much a part of popular culture already that they are virtually guaranteed classic status. Titanic, Toy Story, Silence of the Lambs, Pulp Fiction and dozens of other films will undoubtedly be watched and enjoyed by broad audiences for decades to come.
The cult classic is a particular kind of classic film that gains enduring popularity despite a failure to captivate a wide audience or even, in many cases, to be an artistically competent film. Cult classics are popular with niche audiences, and they retain their popularity over a long period of time, without ever achieving mainstream success. In some cases, cult classics eventually accumulate a broad audience and turn their cult status into an unqualified classic designation.