What is the difference between a cult and an established religion ?
Is it the size of the following ?
Is it the time of origin, i.e. recent vs long ago ?
Is it the number of followers ?
Or is it some combination of the above, or something else altogether ?
A "cult" is a disparaging term for a religion that someone other than yourself belongs to. YOUR belief group on the other hand is obviously not a cult, but a religion.
(Actually I guess it might be size, but in common vernacular I think the above is actually often the case).
For example, most educated Catholics don't consider the Anglican church or mainstream Judaism or Islam as cults. On the other hand, many Catholics would consider Jehova's Witness and some pentecostal churches as cults, even though at some level Catholicism's doctrines share more bullet points with these organizations than with Judaism or Islam.
Size usually isn't a criterion in the definitions used for groups, whether the disparaging ones or the more objective ones (e.g., in the approach used in Marc Galanters Cults: Faith, Healing, and Coercion.)
Edit: My guess substantiated by wikipedia: http://en.wikipedia.org/wiki/Cult
So in effect, there is no fundamental difference.
I think the biggest difference is that cults (as the article mentions) generally are run by people who don't actually believe in what they teach. That does occasionally happen in established religion, but it is not common.
The other thing about cults is that they are mainly about the personal gain of the leaders, while at the same time requiring members to devote much or all of their money to the cult continue to be a member. This is unlike most established religion, which is generally giving is always optional and often more transparent (all of the religious organisations I work with supply full budgets to all the members). Sadly, of course, there are exceptions here too (but those exceptions often are heading towards cultishness).
This sounds impure, but actually as I think about it it's quite scientific: does it lead to mass suicide? Can respectable people hold such beliefs while continuing the behaviors that made them respectable? Is the cult/religion "fit" in the evolutionary sense? By their fruits shall ye know them, it seems.