People argue about whether the major religions of the world are just well-established cults — that, age and size aside, they are no different from any group that follows a spiritual leader.
A cult is group of people defined by a "religious" devotion to something — often a self-appointed leader. Most people view cults as strange and frightening, mostly because cults have, over the years, done some strange and frightening things, including murders and mass suicides. Sometimes, too, you'll see cult used as an adjective to describe something or someone with a small, devoted fan-base. John Water's movies are cult favorites, adored by a select group of film lovers but not by the public at large.