A Cult movie is a film which the studio no longer makes money from but is kept alive by a loyal fanbase.
Star Wars is not a cult film because its still a license to print money. The Blues Brothers, The Ninth Configuration, The Keep are all 'cult' movies but the line gets a little blurred nowadays because studios realise they're 'cult' movies and rerelease them on dvd.
It's overused to the point that movies are called 'cult' before anyone's seen them. Donnie Darko for example was sold as a 'cult' movie because the studio didn't know what to make of it.
I'm sure most people on here could recommend a few 'cult' movies but it's really about movies you love dearly for whatever reason.
I love William Peter Blatty so I can recommend The Ninth Configuration and Exorcist 3 as a double bill. I'm sure you won't be lost for other suggestions around here.