Although there is almost no difference in the flavour or preparation, these are different species. It's hard to define the differences, though, because there is so much overlap. Shrimp are generally smaller and live mostly in salt water, while prawns are likely to be bigger and live in fresh water. If you want to make salt and pepper shrimp, just follow the recipe, but don't overcook them if they are small. Usage differs between the UK and US too.