I always thought "prawn" was the British English word for the animal that Americans call a "shrimp," and although that is the case, there is also actually a scientific difference between a shrimp and a prawn.
While very similar in appearance, shrimp and prawns are two different species, with different gill structures, different habitats, and a difference in taste.
Shrimp come from both fresh and salt water and can live in both cold and warm waters; if they came from cold waters, then they will be smaller in size. There are more saltwater than freshwater species of shrimp.
Prawns come from fresh water, and are much larger and meatier than shrimp, like a langoustine. Their meat is much sweeter than those of shrimp. Prawns also have bigger pincers and longer legs.
And there you have it!
(Image: Legal Seafoods)