The two words are used interchangeably in markets and restaurants everywhere.
Some people say the difference is size.
In many parts of the country, small and medium shrimp are sold simply as shrimp, while large, extra-large, and jumbo shrimp are called prawns. In some areas, all shrimp, small and large, are sold as shrimp, while in other regions, all you'll find are prawns. Sometimes called Florida or Caribbean lobsterettes or French langoustines, these shellfish can be hard to find in markets. And unlike shrimp, Dublin Bay prawns are usually cooked with their heads on. The claws make quite an attractive presentation, although they're too tiny to render any meat.