At its most basic, a cult is simply a small, unestablished, non-mainstream religious group that typically revolves around a single leader. The American Heritage Dictionary defines "cult" this way:  (1) A religion or religious sect generally considered to be extremist or false, with its followers often living in an unconventional manner under the guidance of an authoritarian, charismatic leader. (2) A system or community of religious worship and ritual.

The first definition is closer to the common usage of the term today, but you'll notice there's no mention of brainwashing, murder or mass suicide. There is no meaningful difference between a cult and a religion in terms of faith, morality or spirituality. The primary differences are that a "cult" operates outside of mainstream society, often calls on its followers to make an absolute commitment to the group and typically has a single leader, whereas a "religion" usually operates within mainstream culture, requires varying levels of commitment from its members and typically has a leadership hierarchy that, in practice, can serve as a series of checks and balances.

But destructive cults are a different story. There is a big difference between a destructive cult and a non-destructive religion (or a non-destructive cult). A destructive (or totalist) cult exploits its members' vulnerability in order to gain complete control over them, often using unethical psychological techniques to bring about thought reform. It can be said that a non-destructive religion or cult attempts to alleviate its members' vulnerability through spiritual guidance in an effort to help them exercise control over their own lives....

Many of these religions are founded by a single person who retains a position of exclusive power within the organization, and power tends to corrupt even the most ethical among us. In the case of The People's Temple, there is evidence that its leader, Reverend James Warren Jones, was abusing prescription drugs and becoming increasingly paranoid through the 1970s. Next, because these groups operate outside the mainstream, there is usually no one checking up on their operating procedures, so a corrupt or mentally unstable leader is free to exploit his followers to his heart's content. In addition to this authoritarian leadership structure, some primary characteristics of a destructive cult include:

Charismatic leadership

Deception in recruiting

Use of thought-reform methods

Isolation (physical and/or psychological)

Demand for absolute, unquestioning devotion and loyalty

Sharp, unsurpassable distinction between "us" (good, saved) and "them" (bad, going to Hell)

"Inside language" that only members fully understand

Strict control over members' daily routines