Well, speaking as an unrepentant old atheist, I'd say the difference between a sect, a denomination, a cult, a whatever, is functionally nothing. They're all just superstition. The only practical difference is in the status various societies give them. Denominations, like Catholicism, Anglicanism, Lutheranism, etc., are generally respected and honoured. Sects are generally splinter groups from one of those; there are, for instance, charismatic and fundamentalist groups that call themselves Catholic, Anglican, Lutheran, and so forth, that the officlal church usually looks upon with disfavour, but the state treats as another church, with tax breaks and various other special statuses. And then there are people like David Koresh, and groups like the Branch Davidians, who are clearly whacko; those are the cults.
Far as I can tell, none of them really mean anything, they're all nonsense. The distinctions among them are based on the legal status they have in the societies where they exist. The Bahai faith, for example, is just another church in North America and Western Europe, with the same legal status as the major Christian denominations. In Islamic countries Bahais have been savagely persecuted as dangerous heretics. We in the West would call them a sect, Muslims view them as a dangerous and heretical cult. It all depends on your point of view, and my point of view, just in case you missed it above, is that they're all mystic nonsense.