What is a prawn and a shrimp ?

According to Food and Agriculture Organization (FAO) of United Nations, shrimp is a seawater creature, while prawn's natural habitat is  fresh water. This definition is not used by the grocery stores, fish markets, or the restaurants. Retailers commonly differentiate them by the size. A larger jumbo shrimp (Less than 25 shrimp to a pound) with head-on is normally referred to as prawn.

Scientifically, there is a difference between the two species. Both species are found caught in sea water as well as fresh water with varying sizes.