A cult is a religion without political clout. Or something like that.
I’ve always though that a good benchmark would be size. It’s like gangs: when does a gang become a ruling elite? Answer: when it becomes acceptable to a sufficient number of persons.
I.e.: one could classify Democrats (or Republicans, for that matter) as a “gang” that has sufficient power and authority, bolstered by numbers and supporters, to gain widespread credibility. Indeed, they both are accepted ruling parties in America. But they’re still an amalgamation of gangs. Lobbyists or special interest groups could also be regarded as gangs, if one defines “gang” as a group that desires a certain power within a larger group, also definable as “a bigger gang.”
Same for religion. If a belief has a sufficient number of adherents, it crosses the line from cult to mainstream belief.
Otherwise, religions such as Ba’hai or Christian Science would have to be classified as cults (to me, all religions are cults, but that’s another story. To me, all political parties are gangs. 
The anti-cult movement has co-opted the once relatively benign term of “cult” and manufactured a fear of all new religions. While there is no doubt that certain religious groups can be abusive neither size nor novelty nor leadership nor mysterious practices/knowledges can be used as an adequate delimiter of what the “difference” is because there is no objective measure of these qualities.
My opinion: because “cult” has become such a loaded term either call every religion a cult or call no religion a cult. It’s just been too abused as a classifier in the last few decades.
About a million people.
::: sigh :::
There are four general meanings of the word cult as it is used in American English.
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as a direct cognate translation of the Latin cultus, meaning any well-organized set of religious beliefs.* In this sense, the various sects and denominations of Christianity and Islam could each be called cults.
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any religious group with a strong connection to a living or recently-deceased leader. In this case the LDS could be called a cult up until the death of Brigham Young, at which point the direct connection to Joseph Smith was broken. (On the other hand, Christianity would be a cult of Jesus, in this sense, until the death of the first generation of Apostles. Woudn’t this definition irk some fundies?)
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a nominally religious-based group enforcing control over all aspects of the lives of its adherents in the manner described in the previous posts.
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any religious group opposed by various fundie groups.
*Because of #4 and #3, the RCC gets in trouble every once in a while when some idiotic translator does a word-for-word substitution from some Latin document and the church appears to be accusing Judaism or the Orthodox Christians of being mind-control experts. Given the popular meaning that adheres to the word, I think the translations are bad, but if one is reading one of those documents, it should be noted that the RCC includes itself in the definition–and that formal documents use meaning #1.
As must surely be clear by now, “cult” is something of a term of art – to some atheists, all religions are cults (and some theists see atheists as a “cult”), whereas members of what almost everybody agrees is a cult of course see themselves as members of a religion.
Accordingly, this thread is off to IMHO.
Argh! I was previewing and blew out my post!
This is my area of academic expertise (PhD, sociology of religion), and I came in here expecting to have to straighten you guys out – not necessary. I’m impressed.
For me, the bottom line is degree of tension with the larger society. Cult – high degree of tension; church – low degree of tension. Many groups, as they grow larger, start downplaying the beliefs and practices that piss people off. Thus, some groups that we consider churches (Mormons, e.g.), started out as cults, but gave up practices (like monogamy) to get more acceptibility (like statehood for Utah) and thus became more churchlike.
Keep in mind that some groups prefer to maintain a high level of tension, usually because it suits their sense of identity – I’m thinking of the Amish and the Hasidim here. Both of these groups, however, are actually sects, not cults (yupper, not a clear either-or situation here! We’ve got a third term!). Sects are groups that broke off from a church in their society to return to the original, purer beliefs or practices, beliefs or practices that the sect members see as having been abandoned in the process of moving toward social acceptance. Cults, on the other hand, are of independent origin (usually founded based on the vision of a charismatic leader – a technical term with a specific meaning, by the way), or they might be sets of teachings that are dominant elsewhere but create tension when followed in a new society (e.g., Hinduism or Buddhism, when practiced in the U.S., are often considered cults).
Hope this helps.
I’m not gonna preview, lest I blow out the post again – pls. excuse any typos.
having been in a cult at one time (many years ago, for about 3 years), I’d have to second twickster47’s definition as a set of teachings, often -based- on a “formal” religion but with a charismatic leader, usually one who has their own interpretation of the faith. Sometimes, followers will begin learning about the religion on their own and find disparities between the leader’s teachings and the actual tenets of the faith. Often, when followers leave the cult, they are shunned by those who remain “inside” - if someone has been in a cult for many years, this makes leaving even harder because the only friends/family may have been those others in the cult.
Also twickster, I’d like to point out that the practice the Mormons/LDS gave up was -polygamy- – they went to monogamy. 
I’ll just quote one of my bumper stickers (in my locker at a religious school):
It seems to me-though I am not the expert-that cults are where all the members live together and avoid the public or “non-believers”.
The best definition I have seen is that a cult is something you join, whereas a religion is something you are born into. Owing to the fact that mainstream religions can and do sometimes get people to join them, we could tweak this to say that a cult is a religion in which a majority of the adherents have joined rather than been born into it.
My opinion is that cults not only actively seek to recruit new members, but that they also try to make it difficult for existing members to leave. That, whether through social stigma or actual threats, they attempt to force members to stay. Also, a cult is more centered around a charismatic leader, rather than doctrine that can be studied on it’s merits. Thus, Billy Graham may be charismatic, but those who believe in his preaching will not be forced to stay within the fold if they don’t want to, so he is not a cult figure. Jim Jones was charismatic, but people were not allowed to leave. He was a cult leader. That is one problem I have with the LDS as I understand it - if you choose to leave, people from the church will continue to contact you in an effort to make you come back.
StG
StG
I am surprised no one has yet posted the Advanced Bonewits’ Cult Danger Evaluation Frame.