Tin snips are availiable in a range of sizes, from small to... well, large. They are a lot like scissors, but they are heavier duty and blunter, the handles are further apart, and the jaws are thicker.
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Tin snips are used like big scissors, for cutting through thin (about 1mm) sheet metal, trimming PCBs to size, and suchlike. They aren't terribly sharp, though, so cutting flexible things is hard.