What really is a Cult?

A cult is best exemplified by the Jonestown fanatics. They inculcate their believers in odd religious and moral doctrines, and they isolate them from others. A key defining feature is that there is a penalty for leaving the cult.

The largest and must virulent cult today is Islamism. They carefully isolate their fellow cultists from other Muslims, devoting most of their violence to reformist or secular Muslims outside the cult, and they decree the death penalty to those who abandon Islamism as apostates.

Note that JWs, SDAs, Mormons, and other marginal Christian groups are not cults because they do not prevent anyone from leaving.

Group led by something seemingly true.

Here are several traits of religious cults that the Jehovah's Witnesses and other cults own.

Submission: 

Complete, almost unquestioned trust in the leadership. 

Leaders are often seen as prophets, apostles, or special individuals with unusual connections to God. This helps a person give themselves over psychologically to trusting someone else for their spiritual welfare.

Increased submission to the leadership is rewarded with additional responsibilities and/or roles, and/or praises, increasing the importance of the person within the group.

Exclusivity

Their group is the only true religious system, or one of the few true remnants of God's people.

Persecution complex

Us against them mentality. Therefore, when someone (inside or outside of the group) corrects the group in doctrine and/or behavior, it is interpreted as persecution, which then is interpreted as validation.  

Control

Control of members' actions and thinking through repeated indoctrination and/or threats of loss of salvation, or a place to live, or receiving curses from God, etc.

Isolation

Minimizing contact of church members with those outside the group. This facilitates a further control over the thinking and practices of the members by the leadership.

Love Bombing

Showing great attention and love to a person in the group by others in the group, to help transfer emotional dependence to the group.

Special Knowledge

Instructions and/or knowledge are sometimes said to be received by a leader(s) from God. This leader then informs the members.

The Special Knowledge can be received through visions, dreams, or new interpretations of sacred scriptures such as the Bible.

Indoctrination

The teachings of the group are repeatedly drilled into the members, but the indoctrination usually occurs around Special Knowledge.

Salvation

Salvation from the judgment of God is maintained through association and/or submission with the group, its authority, and/or its Special Knowledge.

Group Think

The group's coherence is maintained by the observance to policies handed down from those in authority.

There is an internal enforcement of policies by members who reward "proper" behavior, and those who perform properly are rewarded with further inclusion and acceptance by the group.

Cognitive Dissonance

Avoidance of critical thinking and/or maintaining logically impossible beliefs and/or beliefs that are inconsistent with other beliefs held by the group.

Avoidance of and/or denial of any facts that might contradict the group's belief system.

Shunning

Those who do not keep in step with group policies are shunned and/or expelled.

Gender Roles

Control of gender roles and definitions.

Severe control of gender roles sometimes leads to sexual exploitation.

Appearance Standards

Often a common appearance is required and maintained. For instance, women might wear prairie dresses, and/or their hair in buns, and/or no makeup, and/or the men might all wear white short-sleeved shirts, and/or without beards, or all wear beards.

The word "cult" has two definitions.

In the strict sense of the word, a "cult" is a subset of a religion that employs a unique set of rites and observances in devotion to a specific deity, saint, or unique representation of a deity. For example, within the Olympian religion of the ancient Greeks, there was a specific religion at Eleusis dedicated exclusively to Demeter. They had their own unique rites, rituals, and ceremonies that were not practiced outside of Eleusis. So, we describe them as "the cult of Demeter at Eleusis." Similarly, there was the "Cult of Zeus Dodona," which was a unique set of rites in Epiris dedicated to Zeus.

Modern examples of this would be the "cult of the Virgin of Guadalupe. The worshipers are Roman Catholics, but they practice a unique set of observances dedicated specifically to the apparition of the Virgin Mary that appeared at Guadalupe. There are many such cults dedicated to specific saints in the Roman Catholic tradition.

A more obscure example would be the cult of Damballah in New Orleans. The adherents, in this case, were members of the wider religion of Haitian Vodou, but they developed an entire religious culture dedicated to the Loa (the Vodou equivalent of a god or saint) Damballah.

So, that is the proper definition of a cult. However, there is a more commonly used vernacular definition of cult that refers to a religious (or sometimes secular) organization that essentially brainwashes its believers into a fanatical devotion to a single charismatic leader. Cults, this sense of the term, frequently isolate members from relationships with non-members, refuse to allow members to encounter non-members without another member present, control the media to which members are exposed, fill up the schedules of members to keep them preoccupied with cult activities and cut them off from "normal" outside life, etc. Some of the aggressive cults may even starve, beat, or otherwise abuse members in order to create a sense of Stockholm syndrome to strengthen their loyalty to the cult.

So, to recap, there are two definitions:

1) A subset of a religion dedicated to a unique expression of religious devotion centered around a specific divine figure.

2) A religious organization that uses aggressive brainwashing and social engineering to instill fanatical loyalty to the cult and its leader/founder.

You will find that many people conflate these two definitions in order to disparage a group. For example, someone may refer to the Southern Baptists as a "cult" using the second definition of the term, and then when a Southern Baptist complains about it, they switch to the first definition of the term and argue (incorrectly) that the Southern Baptist denomination is, in fact, a cult. That, of course, is the fallacy of equivocation.

a cult is normally an offshoot of an already established religion, but has extreme, often damaging views.

examples of a extreme cults are:

Jehovah's witnesses, Christian Science, Mormonism, Baptists, and other Evangelical movements

Reciting 666 rituals for 13 devil calendar months for at least 14 hours a day

A religion that controls the personal lives of its members, financially, time, and relationships (romantic, interpersonal, and familial). It's actually a matter of degree, all religions are cults, just some exhibit more cultlike behavior than others.

A "cult" is a religion that followers of a larger, older religion don't approve of.