Gherkins are pickled cucumbers. You can find plain, sweet and dill varieties of gherkins. A gherkin's flavor depends on how the cucumber was pickled and seasoned.
Cucumbers are pickled using vinegar, water, and salt. A cucumber can be pickled in as few as four or five hours, though some gherkin recipes call for soaking the cucumbers in a vinegar, water and salt mixture for up to 30 days. Some varieties of gherkins can be stored at room temperature in dark cupboards and pantries for up to two years, while others must be refrigerated as soon as the pickling process is finished.