Pickles, dried fruits, cured sausages and fish, dried milk have in common that they involve controlled degradation of an original product to create something that can be stored for much longer, often developing pleasing flavors and textures all their own. Preserving foods is an ancient practice, and very easy to do. There are a few forms of preservation, like pickling, curing, fermentation, and drying. People have also been preserving milk by making yogurt, cheese, and the like, all examples of the controlled degradation of the original food to produce something that can be stored much longer under conditions that would cause the original product to spoil. Drying removes water which makes it difficult for anything to grow. Dried fruits are a clear example of this, and you've seen peppers, spices, and mushrooms that have gone through this process. Dried milk is another good example, since it doesn't require refrigeration. Another form of preservation is fermentation. Pickles, olives, sauerkraut, and preserved lemons etc. fall into this category.
I decided to make preserved lemons, to be used at a future date. To do this, I cut some meyer, eureka, and lisbon lemons into halves and packed them with a 2:1 ratio of salt to sugar. That is all there is to it. Time does the rest. These will sit in their jars to ferment for several weeks.
The idea behind preservation is to deactivate the enzymes in the food while creating an environment unfriendly to sickness-causing microbes. The preserved lemons are also a fermented product, relying on the slow degradation of the lemon flesh to create an intensely flavored product which can be used anywhere lemons are called for, and more. In the preservation of lemons, the acid, salt, and packing creates an oxygen poor environment, which inhibits bacterial growth while promoting the fermentation of the flesh. By destroying the living tissue of the food, this makes it unavailable to disease causing bacteria and molds, thus preserved foods can be stored indefinitely.
(The above is based on a quick reading of McGee)