Crabs are crustaceans of the subclass Brachyura, which usually have a very short protruding "tail" (abdomen) and are usually completely hidden under the chest. They live in all of the world's oceans, freshwater and land, usually covered with thick exoskeletons, and have a pair of claws. Many other animals with similar names (such as hermit crab, king crab, porcelain crab, horse and crab louse) are not real crabs.

Crabs are usually covered with thick exoskeletons, mainly composed of calcium carbonate, and are equipped with a pair of claws (claws). Crabs are found in all the oceans of the world, and many live in fresh water and on land, especially in the tropics. Crabs vary in size from pea crabs a few millimeters wide to Japanese spider crabs with legs up to four meters (13 feet) long.

About 850 species of crabs are freshwater, terrestrial or semi terrestrial. They are found all over the world in the tropics and subtropics. They used to be considered a monophyletic group, but now they are considered to represent at least two different lineages, one in the old world and one in the new world. [