Cults can occur inside of and outside a 'mainstream' religion (here I take it that the question is referring to the classical 'cults' whereby strong leaders have control over members of their group, rather than alternative definitions such as 'personality cults', or 'cults of devotion').
Three ideas seem essential to the concept of a cult.
1. Thinking in terms of us versus them with total alienation from "them."
2. The intense, though often subtle, indoctrination techniques used to recruit and hold members.
3. The charismatic cult leader. Cultism usually involves some sort of belief that outside the cult all is evil and threatening; inside the cult is the special path to salvation through the cult leader and his teachings.
The indoctrination techniques include:
Subjection to stress and fatigue;
Social disruption, isolation and pressure;
Self criticism and humiliation;
Fear, anxiety, and paranoia;
Control of information;
Use of auto-hypnosis to induce "peak" experiences
Cults are absent of the betterment of the individual person but rather than leader only. Cults try to subvert the human will with total and complete obedience to the leader of a group or sect
The term 'cult' tends to be used as a term of abuse. As shown above, there are stringent guidelines for defining whether a body of people (whether religious, lifestyle orientated, political) are a cult or not. An example of of cult outside of 'mainstream' religion is David Koresh. Within 'mainstream' religion we have the cult around Chris Brain at the 'Nine o'clock service' in Sheffield, within the Anglican church.