A religious/non religious group that follows a series of strict beliefs, may include worshiping a specific God/Deity or multiple Gods/Deities, or following strict specific ideals. May involve some form of brainwashing that their knowledge is correct and that everyone else is wrong, WILL have a hierarchy, and may be lead by one of a small group of Charismatic leaders, and typically will shun those who are ex-members.
It's the One True Religion! (I hear this from the Big Three Religions)
If you don't believe in God you are going to hell!
I've just been initiated into this super secret Order/Coven.
Yeah we are a cult, but we believe in peace and love, not fear and hate.
Not all of the above will apply to a cult, but at least one of the descriptions will. A Cult isn't necessarily Good or Evil, it depends on how the cult leaders use the power they have.