It can be diffi-cult (pun intended) to define "cult", as well as "religion", for that matter. However, I think what comes to most people's minds is a new sect that claims to promote love, purity, or good order, but is harmful. Cults are often syncretistic or they claim new revelation or insight, and they are marked by false doctrine commonly accompanied by greed, sexual immorality, and murder and abuse (physical and emotional). Cults are born and fueled by sinful hearts and demonic forces.
Two things I think are worth considering is:
(1) There is a point where a cult may grow large enough to be commonly perceived as a religion. I would argue that Scientology and Jehovah's Witnesses fall into this category. JW, as I understand it, is less aggressive than most cults, but I still consider it a cult. I realise it's not a popular opinion. You could even argue that all false religions are cults. But sometimes it may be helpful to distinguish between a historical religion and a new sectarian group.
(2) Historically, a cult tended to be centred around one person or a small group of people. I think in our time, we're seeing something new in this respect, in that cults can grow organically online and through social media — their doctrines are being developed sporadically by different individuals. I'm of the view that the transgender movement is a kind of cult.
Many cults have come out of the church. There have been many anti-trinitarian sects throughout church history. Some of those have in recent years found a way back into the visible church under the guise of non-denominationalism, so that's something to watch out for as well. That is, just because a congregation has the appearance of a charismatic church, doesn't automatically mean that it is a charismatic church.
Where I live, in Japan, and in Asia in general, there are many cults. Most of them are syncretistic with a strong emphasis on self-help or self-actualisation, so cults are very common in our day. But I think to many the word "cult" is so strong that we'd prefer to soften our language in order not to offend. However, I think if the fruits of the sect are cultish, we shouldn't shy away from using the term. It's possible to criticise a sect and its beliefs and practices, and at the same time have compassion on its adherents who are caught up in it.