A gherkin is a variety of cucumber : the West Indian or burr gherkin (Cucumis anguria), which produces a somewhat smaller fruit than the garden cucumber (Cucumis sativus).
Gherkins are cooked, eaten raw, or used as pickles.
Gherkins are usually picked when 4 to 8 cm (1 to 3 in) in length and pickled in jars or cans with vinegar (often flavoured with herbs, particularly dill, hence, "dill pickle") or brine.