The cold pink prawns seen in shops are not 'fresh', but cooked. The boiling is done on boats at sea as soon as the prawns are caught to maximise quality. Grey-coloured 'raw' prawns are usually best purchased frozen, unless you are buying direct from a fisherman.
Whole prawns offer good value for money because the shells and heads can be used to make stocks and fish soups.
Large 'raw' prawns are best de-veined before cooking. To do this, peel away the shell, then cut along the back with a small sharp knife and pull out the dark thread.
There are innumerable culinary uses: prawns are a favourite of Asian cooking, used in all manner of spiced dishes as well as stir-fries and pasta sauces. Plain-boiled prawns make terrific sandwiches and salads, while raw prawns are exceptionally good for barbecuing and grilling.