If I am going to comment in regards to "cult films", I will upfront and let you know I speak within the horror genre only. Why the horror genre? Because that is my preference, and speciality.
Within the horror world, cult films have been rewarded with such greats as, Night of the Living Dead; Texas Chainsaw Massacre; Dracula; Shining; Exorcist, I Spit on Your Grave; Blair Witch and many more.
Why are these films considered cults?
Because of the followers. The people who will slap down money at the ticket boothes to catch them at all night horror runs, or at the local video store, no matter how many times they have seen the movie.
These are the movies that have forums pertaining to just them, and websites devoted to them. There is "toys" that folks will collect pertaining to the film. (I am guilty of purchasing anything related to quite a few films as such)
These films bring people together at conferences. A full night conversation can result from just one cult film. There are panels devoted to these films.
I remember this year at the WHC that one of the panels discussions ended up becoming a free for all pertaining to just these type of cult films and the people who are devoted to them....and I think that is the key word, devoted.
So to wrap it up quickly instead of going on and on about my favorite genre, "what is a cult film"?
A flick that folks are devoted to, talk about, remember in favor and will hold something special to them...may it be the blood, gore, suspense or characters