A nutcracker is a tool designed to open nuts by cracking their shells. There are many designs, including levers, screws, and ratchets. The lever version is also used for cracking lobster and crab shells.
Lever nutcrackers with hazelnuts
Screw nutcracker with walnuts
A Crackerjack nutcracker, which uses a ratchet
Cracking a crab claw
A nut is a fruit consisting of a hard or tough nutshell protecting a kernel which is usually edible. In general usage and in a culinary sense, a wide variety of dry seeds are called nuts, but in a botanical context "nut" implies that the shell does not open to release the seed (indehiscent).
Chestnuts are both botanical and culinary nuts.
Nuts being sold in a market
Raw mixed nuts, sold as a snack food
Image: Almonds in shell, shell cracked open, shelled, blanched