What is a "Scam" Site?

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My interest is specifically PTC sites, but I am sure the question applies to most online money making sites. I see the word "scam" thrown around a lot and I am not always sure what people mean by it. At the end of the day, it seems that it just means, if you do the work or pay money, the site won't pay you. What's weird is that a scam to me suggestions an intention by the owner of the site to defraud you. But what about the sites that are run by inexperienced, incompetent admins, is that a scam? How about someone trying about a payment scheme that ends up not working, etc... Anyhow, what do y'all think a "scam" site is.