In the US and Britain large shrimp are often called prawns. In India all shrimp are called prawns - which also seems to be the norm for Australia. In some parts of the world prawn is used to denote a fresh water shrimp - the word shrimp being reserved for it's salt water cousins. In other places - the words are used interchangeably for the same thing. I've gotten bags of dried shrimp from my local Asian market and some are labeled as shrimp, some as prawns, some as shrimp (prawns), and some as prawns (shrimp) - they were all smaller than my thumbnail so in that case size wasn't a factor.
One other critter worth mentioning, just for the fun of it, is the Dublin Bay Prawn. I ran across it after something someone somewhere else said that "shrimp scampi" was a redundant misnomer since scampi in Italian means shrimp - thus it translates to "shrimp shrimp". Actually - this may be a form of Italian truth in advertising - indicating you're getting a dish with large shrimp the size of a scampi. Dublin Bay prawn = Dublin prawn = langoustine = scampi = Norway lobster. Although they look similar to a large shrimp, but with prominent fore claws, they are in fact a small lobster.
Throwing off the commercial, culinary and cultural habit/linguistic confusions ... yes, there actually is a difference between prawns and shrimps biologically - which appears to be primarily the differences in the gills. Here is probably more information than you ever wanted to know about prawns
, and scampi
/ Norway lobster
I would suggest that the next time you see prawns and shrimps of the same size being sold side by side and you want to know the difference - ask the person selling them. They will be able to tell you how they
define the difference.