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A thief is one who steals your things without your knowledge.

A robber threatens you and grabs your valuables forcibly.

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Thief ---> One who steals, esp. without force or violence; one who commits theft..A thief is a larger category a set which would include both the subsets of robber and burglar, among others.

Robber ---->Robber uses force or a weapon,
A robber is someone who uses force or violence in order to steal something.