Historically brining has been used as a method to preserve meat.  A brine has a very high salt to water combination then the addition of sugars, spices, and other ingredients are added for giving the meat flavor.  This process, also know as curing, binds the water to the meat or completely removes it so its not available for food-spoiling organisms.  With the invention of the refrigeration, traditional brining is less popular for food preservation, but still is used with some meats.

Today brining is used to improve the flavor, texture and moisture content of lean cuts of meat.  This is achieved through soaking the meat in a fairly salt water solution any where from an hour to many days, depending on the size of the meat.  The good part about this type of brining is if you overcook the meat a little bit, it will not completely dry it out.  It provides an extra little cushion for creating fabulous dishes.

At times a curing agent is added to the brine, such as Morton Tender Quick, which allows you to cook or smoke foods below 140 degrees.  This curing agents prevent the growth of botulism.

Scientifically, brining uses the principles of diffusion and osmosis, for more information check out: Brining.