Help

I need help with an element of internet discourse. People on the internet seem to criticize things for being "Manichean" almost as regularly as they do for being "straw men" or other logical fallacies. The logical fallacies kind of irk me, but I understand what they all mean (and people tend to use them in a fairly consistent manner, though I very often disagree with their application and never understand why so many people think simply calling something a logical fallacy means that they've won an argument). "Manichean" I don't understand, and people seem to throw it around willy-nilly in any ol' context at all. I have a vague understanding of Manichaeism, the Gnostic belief that involves a kind of dialectical teleology (how's that for pretentious talk!) in which the good in the world is gradually replaced by the evil (or something like that). I don't know what kind of behavioral demands arise from this belief, though I'd be interested to because in general the Gnostics are pretty crazy and fascinating. But anyway, this vague knowledge does not at all help me to understand what the hell people are talking about when they call people/behaviors Manichean, or why it's a criticism. Can someone help me?