Well, there is in fact a difference, but it's a subtle one. In general terms, a gherkin is simply a small type of cucumber used specifically for pickling purposes. A cornichon is also a gherkin ...

A true dill cucumber is between the size of a gherkin and a small cucumber, but if you can’t get hold of any, you can use gherkins or small immature cucumbers instead. You will need two 500 ...