I had always thought "prawn" was British English and "shrimp" was American English, but I looked it up, and it seems there is a difference in meaning. http://www.diffen.com/difference/Prawn_vs_Shrimp

Sometimes it depends on who you ask. Australians typically say "prawn". If you say "shrimp" to an Aussie, we think you mean something that looks like a prawn but is actually quite small. The prawns we eat are typically large. "Shrimp" can also be used to describe a small and weak person (and unkindly so).