The problem with trying to define the notion of a "cult" is that to some degree the definition depends on who is doing the defining. It is a generally pejorative term. No one is likely to call their own religious denomination or a religion they admire a cult; instead, "cult" is often a term people use to disparage other people's religious beliefs. For example, some evangelical Christians refer to the Church of Jesus Christ of the Latter Day Saints (the Mormons) as a cult, but members of the LDS church would not talk about themselves as belonging to a cult.
The word "cult" itself derives from the Latin "cultus" which means something that is cared for or tended (this word is also the root of "cultivate", which has a close English sense to the Latin term). In antiquity, one "cared for" or worshiped gods by tending their shrines and and performing regular acts of worship (such as reciting prayers) or sacrifice (placing, for example, offerings of honey cakes on altars). Thus the religious sense of cult refers to the sense of active worship of a deity. The term took on its negative aspect when Christians began to distinguish pagan "cults" from their own religion.
In popular culture, especially, the term cult is often applied to small, insular religious groups that seek to detach their members from connections with the outside world. They are often accused of brainwashing adherents. Often cults have single charismatic leaders, are strongly authoritarian, and regulate every aspect of their members' lives. Of course, as this is also true of most monastic orders, the difference between a group committed to spiritual and ascetic practices, such as many established Buddhist or Christian monasteries, and a cult often depends on whether the order is accepted by the society in which it belongs or not.
Cults are a distinct form of religion, and in today’s society the word has negative connotations (due to conflicts with the general population involving legal interpretations). Basically a cult is a gathering of persons following the teachings of a charismatic leader, and usually living in a communal society that excludes non-followers; in this interpretation, Christianity, at least in its foundational beginnings, would fill the definition of a “cult”, as would Islam (followers of Mohammad) and to an even greater extent, Mormonism. The main features of this cultist form of religion (as opposed to such religions as Confucionism, Buddhism, Judaism, and Taoism) are:
1. a charismatic human leader
2. a living arrangement in which personal rights are subsumed under group behavioral “rules”
3. a document from which the leader claims his authority and which spells out the rules of behavior (subject to the leader’s interpretation)
4. a policy of exclusivity that does not allow “outsiders” to participate or interfere
5. the sacrifice of any personal wealth to the cult’s leader
6. the suspension of social equalities, especially gender roles
The geographic site where the group lives and worships is often at odds with the general population surrounding it (resulting in legal, even armed, conflict); the source of income for the group is often questionable, sometimes considered marginally illegal. Note that in this definition is very little attention to “religious” matters (salvation, good behavior, personal relation with God, etc.). Cults use a tyrannical, dictatorial form of government, the opposite of democratic.