The difference between prawns and shrimp only exists in some regions. The terms have no concrete scientific meaning, they're both vernacular and depending on where you are both or either term may be used for the same critter. I presume you mean that prawns are the bigger ones, but I've bought local caught fresh "shrimp" on South Carolina coast that were nearly as large as lobsters. There's really no consistent widely respected rule for which is which.
From a purely scientific naming, there actually is a difference between shrimp and prawns. They're different sub-orders. Prawns are Dendrobranchiata, while shrimp are Pleocyemata. I'm not sure it makes much culinary difference to me though.
