Prawns are crustaceans of varying size with sweet, firm, meaty flesh. Cold-water prawns (which are the smaller, more standard prawns) are usually peeled and cooked. Good for both curry and fry.
Generally, the larger prawns are more expensive simply because they are easier to peel and cook. Some people say they have a better texture and taste.
The history of the prawn as a food source is an interesting but short one, as it was not a big part of the human diet until the 1950s. but nowadays they are very commonly cooked.