Alas, there is no absolutely correct answer. In common usage the names are interchangeable, and there are lots of prawn-like or shrimp-like animals to confuse the novice rockpooler. Marine biologists will use the scientific names.

However, the casual rockpooler wanting to put a name to what he finds can follow the terms used for British species only:

The Prawn is transparent with long legs and is found amongst the weed in rock pools on the shore. There are several different species. In the south and west the species in the pools are called by the scientific name of Palaemon.

The Shrimp is the sandy coloured crustacean that lives in the sandy shallows. It has short legs and buries itself in the sand. It is called by the scientific name of Crangon and is found all around the British coast.