The crab (pángxiè) belongs to the class Mollusca, order Decapoda, and is a crustacean whose body is protected by a hard shell and which breathes through its gills. In biological taxonomy, it and shrimp, lobster, hermit crabs are the same kind of animal.

The vast majority of species of crabs live in the sea or offshore areas, with some inhabiting freshwater or land. Common crabs include pike crabs, distant sea pike crabs, green crabs and Chinese mitten crabs (river crabs, hairy crabs, and clear water crabs).

The body of the crab is divided into cephalothorax and abdomen. The back of the cephalothorax is covered with cephalothoracic armor, the shape of which varies with species. The center of the forehead is equipped with the first and second pairs of antennae, and on the outside are compound eyes with stalks. The mouthparts consist of 1 pair of large jaws, 2 pairs of small jaws and 3 pairs of jaw feet. Cephalothoracic armor flanked by 5 pairs of thoracic feet. Abdomen reduced, flattened and curved on ventral surface of cephalothorax. Male abdomen is narrow and long, mostly triangular, with only the first two pairs of appendages deformed into copulatory apparatus; female abdomen is broad, with one pair of double-branched appendages in each of segments 2-5, densely covered with bristles, used to hold eggs. Most crabs are marine, with the most tropical shallow sea species.