Both fermenting and pickling are ancient food preservation techniques. The confusion arises because the categories actually overlap with each other. Some fermented foods are pickled, and some pickles are fermented.
A pickle is simply a food that’s been preserved in a brine (salt or salty water) or an acid like vinegar or lemon juice.
A fermented food has been preserved and transformed by benign bacteria. Usually, that means that the sugars and carbohydrates present in the food have been eaten by the good bacteria (often lactic acid bacteria). The bacteria then convert that sugar into other substances, like acids, carbon dioxide, and alcohol. Those substances, in term, preserve the food (and add to its flavor). So when you eat, say, kimchi, you consume the flourishing colony of good bacteria that has preserved the cabbage for you. Circle of life.
Where they overlap: You know how we just said that a pickle is just a food that’s been preserved with a brine? Well, many fermented foods start with exactly that: A brine. So they’re also pickles. Fermented pickles. Sauerkraut, for instance, is made by packing cabbage with salt and letting it ferment. Traditional dill pickles are made by fermenting cucumbers in salty water. Kimchi can be made with a bunch of delicious things, like cabbage, radish, garlic, anchovy and chile, but salt is the essential.
Why do traditional dill pickles and sauerkraut taste so tart if they’re not made with vinegar? One of the substances bacteria produce during fermentation is acid. That means that many fermented foods end up tasting very acidic.
But not all fermented foods are pickles! For instance, you would never think of sourdough bread, or beer, or yogurt as a pickle.
And: Not all pickles are fermented! You can also make pickles by pouring hot vinegar over vegetables. Those quick pickles, which include many commercial varieties, are usually not fermented.
The upshot: Fermented foods have lots of proven health benefits thanks to good bacteria, and the fermentation process also results in wonderfully complex flavor. When you can, seek out artisanal producers that make pickles the old-fashioned, fermented way. They’ll taste better and be better for you. You can also make them yourself.