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I think it's set before the HP films but isn't, strictly speaking, a prequel - we won't see any recurring characters, or repeated plot points, or similar.
There's a book of the same name, but I think that's more a Monster Manual of the HP world, rather than a novel from which this film is derived. (Oh, and this is supposed to be the first of the trilogy of films. But, unlike the Hobbit, they haven't made three films by splitting up one book.)
I expect it will do very well, but it just won't be as well received as the HP films - much like the SW or LotR prequels.
It's set in the 1920s or 1930s. There will be appearances by characters who were mentioned in the Potter stories, but I don't think that there are any characters who actually appeared in them, in "Fantastic Beasts."