The names "shrimp" and "prawn" are often used interchangeably. Understandably so. Shrimp and prawns have tons in common. They are both crustaceans, they both have 10 legs, they are both found in salt and fresh water, and they both live near the floor of whatever body of water they inhabit.
Yet, biologically speaking, they are different animals. Things labeled prawns are often larger, but aren't necessarily true prawns. And plenty of shrimp aren't shrimpy in size at all. Want to get really confused? Some "prawns" - such as spot prawns - are biologically shrimp, and some "shrimp" - notably ridgeback shrimp - are technically prawns. So what is the difference?
Shrimp have branching gills, a side plate that overlays segments in front and behind, and carry their eggs outside of their bodies beneath their tails.
Prawns have lameller gills, side plates that overlap tile-like from front to back, and carry their eggs inside their bodies near their tails.
Are you thinking what I'm thinking? That those differences may seem fascinating and important to a marine biologist but that they mean next to nothing to me? Well, you wouldn't be that far off.
While different varieties of shrimp and prawns vary in taste and texture, those differences do not divide along "shrimp" and "prawn" lines. In short, while shrimp and prawns are not the same, they are interchangeable in the kitchen. So choose your shrimp or prawns based on how they taste, what size you want, and if they've been caught or raised in an environmentally responsible way. (On that last note, see Seafood Watch for more information.)
Learn about American shrimp varieties here.