Crabs are members of the ten-legged crustaceans of upabangsa (infraordo) Brachyura, which are known to have very short "tails" (Greek: brachy = short, ura = tail), or whose stomach (abdomen) is completely hidden under the chest (thorax ) The crab's body is protected by a very hard shell, composed of chitin, and armed with a pair of claws. Ketam is another name for crab.
Crab is found in all of the world's oceans. There are also freshwater and land crabs, especially in tropical regions. Rajungan is a crab that lives in sea waters and rarely rises to the beach, while yuyu is the crab of freshwater inhabitants (rivers and lakes).
