I always look at it like this:
If you walk in to a store looking for a knife you might find it in the Kitchen department or in the Hardware department but if you were looking at a knife you might categorize it as a knife and metal.
Thus you could then set filters saying you are looking for metal knives (as opposed to ceramic or plastic) but you want ones that would be for use in the Kitchen and not in your garage.
So department is where you would find it; category is how you would describe it.
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