Webster’s defines cult as great devotion to a person, idea, object, movement, or work (as a film or book); especially: such devotion regarded as a literary or intellectual fad.

So how do you identify cult films? The simple, but not very helpful answer:

You’ll know it when you see it.

3/4 of the population doesn't get the point of the film, or

A film watched by 100 people 10,000 times.

There are no universally agreed upon guidelines for cult films, but there are some generalizations that most people won’t grumble too much about. For starters, cult films typically fall into one of the following four categories:
 

Good, but under-appreciated:
 

It may be an indie film with no big names (like Withnail and I) or just too damn quirky or surreal for most people (Jacob’s Ladder or Series 7: The Contenders). Either way, it probably means it didn’t hang around the local theatre for very long – assuming it made it there at all.

Limited access:
 

Sort of an extension of the above. Typically foreign or straight to video. (Subway or Diva or Jesus of Montreal… I seem to have a thing for French cinema.)

So bad it’s good: 
 

Sometimes it’s bad as in campy or kitchy (think Barbarella, Flash Gordon or The American Astronaut), sometimes it’s just plain bad (Showgirls).

Mainstream movies that break the rules:
 

It may be well-known and widely watched, but there’s something… some je ne sais quois… that separates it from your basic blockbuster and allows it to attain cult status. (I suppose Pulp Fiction might fit in this category… though it had plenty of quirks. The Princess Bride might be a better example, or some people have even suggested The Lord of the Rings.)

And if you need a bit more to go on, here are some signs that it might be a cult film:

~ It is oft quoted out of the blue (“It wasn't lies, it was just bullshit.” - Elwood in The Blues Brothers) and some people have memorized entire scenes from the movie (as from Monty Python and the Holy Grail).

~ People will stay up until 3:00 a.m. to watch it on TV even though they already own the video. (Highlander comes to mind.)

~ It elicits a fiery passion in its devoted fans, who have seen the movie 108 times. (Take your pick… for many it would be The Rocky Horror Picture Show, for others possibly Star Wars, for me The Terminator and Aliens – I confess, I’m a James Cameron junkie.)

~ Even though it’s 20 years old, there are plenty of movie theatres that not only show it but will actively encourage costumes and audience participation (Rocky Horror again, and nowadays even The Sound of Music).

Then there are also certain directors that are identified as cult (listed with my fave film of theirs):
 

Stanley Kubrick (A Clockwork Orange) David Lynch (Blue Velvet) Coen brothers (Raising Arizona) Quentin Tarantino (Pulp Fiction) Hal Hartley (Trust) Kevin Smith (Dogma) Terry Gilliam (do I hafta pick just one… Brazil, I guess)

And plenty of others like Ed Wood or Roger Corman, but they’re not to my tastes so I’ll leave you to discover them on your own.

Some people have further requirements. Like it should be low budget (think Billy Jack). Or that it should have no sequels, because sequels denote success (which I would argue with, because it would rule out the aforementioned Billy Jack). Some would say that it’s gotta be a film where you either love it or you hate it (so Magnolia and Moulin Rouge might get cult status on those grounds). And lots of other qualifiers that become more and more subjective. You'll have to decide where you draw your own line in the sand.

Being familiar with the movies that many people consider to be cult classics will help you set your own guidelines. A few that bear mentioning:
 

Reefer Madness (1936) Plan 9 From Outer Space (1959)  The Manchurian Candidate (1962) Harold and Maude (1971) Don’t Look Now (1973) Logan’s Run (1976) Mad Max (1980) Blade Runner (1982) Repo Man (1984)

And the list could go on and on. 

As for me, I’m particularly drawn to the quirky stuff. Though technically it's all quirky, I guess... hmm... maybe I'll just list some of my faves that I haven’t mentioned yet:
 

The Adventures of Baron Munchausen Dark City Donnie Darko Erik the Viking Fight Club Heathers Lock, Stock and Two Smoking Barrels Memento The Professional Run, Lola, Run Time Bandits

And if you’re interested in reading up on cult films, try one of these books:

VideoHound's Cult Flicks & Trash Pics
Cult Movies: The Classics, the Sleepers, the Weird, and the Wonderful

So happy, wacky viewing!