Pickling preserves food by impregnating it with acid. Food can be pickled by submersion in vinegar or another acidic liquid or by creating some type of environment that encourages fermentation.
When preserving food in vinegar or other acidic liquid, the item is generally precooked or soaked in a brine for a short time to draw out excess moisture before the pickling process proceeds. The food is then placed in vinegar or other acidic liquid to cover. Cucumbers (in the form of sweet and sour pickles and cornichons, or tiny French pickles), onions, and other vegetables are some of the foods typically preserved in acidic liquids.