Pickling brine is a moderately strong acid which inhibits the growth of microbes; a high content of salt and/or sugar often enhances this effect. Some foods have a high water content, in which case methods must be used to ensure that the brine is strong enough to function properly.
These three examples cover the range of methods used to produce acidic pickling brine. In the case of cucumbers, vinegar (acetic acid) is usually added to provide all or most of the necessary acidity. Sauerkraut relies on a fermentation process to produce lactic acid, so that careful control of temperature and salt content are crucial to the recipe. Fresh olives are naturally quite acidic, and so acid must be removed before they become palatable; depending upon the process used, several batches of brine might draw acid from the fruit and be thrown away before the final batch is added for storage.
It is widely believed that sushi has its origins in a method of pickling. Its defining ingredient is rice flavored with a brine of vinegar, sugar, and salt.