Why are some foods preserved, and what causes food to rot?

Allow me to answer your question by first addressing why foods rot. There are two major factors in the rotting process. One of these is the fact that foods can get "old." In foods like fruit, vegetables, and meat, cell activity has almost entirely come to a halt. These foods are really just parts of a larger organism-- be it a cow or an apple tree-- so when we isolate them and cells begin to degrade, no new cells are being generated to replace them. Light, air, and moisture can all speed up the process of cell degradation, but even in the most protective of conditions, foods will eventually break down at the cellular level. Most foods contain enzymes which enable oxidation and cell degradation.

The second major factor in rot is bacteria! When microorganisms are introduced to a food source, they begin eating away at it. Some microorganisms, like yeast or mold, create colonies on the surface of foods exposed to open air. Others are able to permeate the entirety of a food, especially if this food material is surrounded by liquid. 

Not all rot is bad, though. In fact, some forms of rot are a means of preserving food. Fermentation is the result of bacterial activity and creates a controlled state of rot, allowing the food to be kept longer. Beverages like beer and wine are the result of the fermentation of fruit or grain and can keep for a very long time. Fermented cabbage, like kimchi and sauerkraut, are also well-preserved and keep for a long time. 

Preservation can be achieved through other means, like salting, drying, and pickling. In these cases, the purpose of preservation is to prevent rot by making the food inhospitable to microorganisms.