Why would a plant be categorized as a weed?

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When a plant is in a place where it is not wanted, it is categorized as a weed.

www.dictionary.com - A plant considered undesirable, unattractive, or troublesome, especially one growing where it is not wanted, as in a garden.

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How does a weed differ from a plant?  Why would a weed be categorized as a weed?