As an English person, we use the terms to describe the size. Shrimp being small and Prawn being large.
However according to experts the terms can be interchangeable for example:
"The terms shrimp and prawn have no definite reference to any known
taxonomic groups. Although the term shrimp is sometimes applied to
smaller species, while prawn is more often used for larger forms,
there is no clear distinction between both terms and their usage is
often confused or even reverse in different countries or regions."
According to the linguist Anatoly Liberman:
"No Germanic language
associates the shrimp with its size"
However the exact definition in most dictionaries seem to be a large shrimp.
See wikipedia, I did :)