I quite like this description from the AMC FILM SITE of what makes a cult film:-
"Cult Films have limited but very special appeal. Cult films are usually strange, quirky, offbeat, eccentric, edgy, oddball or surreal with outrageous, weird, unique characters and/or plots. They are often considered controversial because they step outside standard narrative and technical conventions. They can be very stylized, flawed or unusual in striking ways and may also have bombed at the box office."
What this description goes to show is that the definition of what is or is not a cult film is maybe a generational thing and that people from my generation would possibly agree with some of my choices below of examples of cult films and examples of non-cult films and those people from a younger generation would maybe disagree with the choices.
Not that it really matters one way or the other and it can be safely said without fear of contradiction that there is no hard and fast rule nowadays as to which film is and which film is not a cult film.
A small selection of CULT FILMS for me include / do not include:-
SCI-FI - BLADE RUNNER, PLAN 9 FROM OUTER SPACE, GALAXY QUEST, DARK STAR, BARBARELLA, MAN WHO FELL TO EARTH, THEY LIVE, FORBIDDEN PLANET, TERMINATOR, PREDATOR, REPO MAN etc. = cult
STAR WARS, 2001, AVATAR, ROBOCOP, CLOSE ENCOUNTERS, E.T., PLANET OF THE APES, T 2, PREDATOR 2, JURASSIC PARK, any STAR TREK film etc. = not cult
MUSICALS - ROCKY HORROR, QUADROPHENIA, SHOWGIRLS, THE WALL etc. = cult
MARY POPPINS, THE SOUND OF MUSIC, MAMMA MIA! and nearly everything from the 1950s = not cult.
WESTERNS - JOHNNY GUITAR, any Spaghetti or Paella Western in particular ONCE UPON A TIME IN THE WEST and the Dollars Trilogy = cult
HIGH NOON, SHANE, GUNFIGHT AT THE O.K. CORRAL and classic Westerns in general, anything with John Wayne apart from THE SHOOTIST (his last film and one of his best) etc. = not cult.
HORROR - Largest category for cult films with just a few of particular interest - FREAKS, ERASERHEAD, CARNIVAL OF SOULS, CITY OF THE DEAD, NEKROMANTIK, ZOMBIE FLESH EATERS, THE WICKER MAN, THE TOXIC AVENGER, RE-ANIMATOR, SLITHER, THE BEYOND, TEXAS CHAINSAW MASSACRE, Vincent Price E.A. Poe films, DEEP RED, DON'T LOOK NOW, REPULSION, MARTIN, CANNIBAL HOLOCAUST, PEEPING TOM, EYES WITHOUT A FACE, WITCHFINDER GENERAL, SEVERANCE, DEAD SNOW, CABIN FEVER, DOG SOLDIERS, HORROR EXPRESS, horror anthologies in general particularly ASYLUM, DEAD OF NIGHT and FROM BEYOND THE GRAVE, LES DIABOLIQUES, NIGHT OF THE LIVING DEAD (1968), THE INNOCENTS, PSYCHOMANIA, A CLOCKWORK ORANGE, CHRISTINE, THE SHINING, CAT PEOPLE etc. = cult
ALIEN, THE THING, HALLOWEEN, THE SILENCE OF THE LAMBS etc. = not cult
COMEDY/DRAMA - DAZED AND CONFUSED, DEATH RACE 2000, TIME BANDITS, Monty Python films, Mel Brooks films, THE BIG LEBOWSKI, HEATHERS, CLERKS, OFFICE SPACE, SHAWSHANK REDEMPTION, GET CARTER, BIG TROUBLE IN LITTLE CHINA, LABYRINTH, THE PRINCESS BRIDE, DONNIE DARKO, HAROLD AND MAUDE, EASY RIDER, Pixar animation, AMERICAN GRAFFITI etc.
TITANIC, GONE WITH THE WIND and any other epic, THE ITALIAN JOB, Disney animation (apart from FANTASIA) etc. = not cult.
In conclusion (again from the AMC site) an article that I feel describes it well:-
"Many cult films fared poorly at the box office when first shown, but then achieved cult-film status, developing an enduring loyalty and following among fans over time, often through word-of-mouth recommendations. Cult movie worshippers persuasively argue about the merits of their choices, without regard for standard newspaper or movie reviews from critics. THERE'S NO HARD AND FAST RULE TO CHECKLIST TO GAUGE WHAT MAKES A CULT FILM. A CULT FILM IS OFTEN DESIGNATED AS SUCH "IN THE EYE OF THE BEHOLDER" WITHOUT FULFILLING ANY DEFINITION. IT"S OFTEN A MATTER OF OPINION AND ONE VIEWER'S CULT FILM MAY NOT BE JUDGED THE SAME BY ANOTHER VIEWER.
Cult films elicit a fiery and intense passion in devoted fans, and may cause cultists to enthusiastically champion and become devoted to these films, leading to audience participation, fan club membership, and repetitive viewings and showings at repertory cinemas. Cult films have tremendous followings with certain groups, e.g., college campuses, 'midnight movie' crowds, independent film lovers, etc.
The first 'official' midnight movie was Alexandro Jodorosky's strange El Topo (1970, Mex.) (aka The Mole) - a mystical 'spaghetti western' about a black-clad rogue gunfighter on a quest to defeat the 'four masters of the gun.' It premiered at midnight in a rundown NYC theatre (on lower Eighth Avenue) and ran seven nights a week for many months. The concept of long-playing, taboo-breaking, eccentric midnight movies designed to appeal to urban film fans was thereby born."
No need to comment on my choices of cult and non-cult films as it's subjective (see blue bold).