Brining will make your turkey moister by imparting flavored liquid on a cellular level to the muscle tissue of the meat before cooking. This is done via the process of osmosis, by allowing the cells to hold on to the water while they are cooked, via the process of denaturation. The proteins coagulate, forming a matrix that traps water molecules and holds them during cooking, preventing the meat from dehydrating – particularly useful for turkey since the breast meat often dries out. The salt is also desirable as a preservative. The flavor in the brine ends up flavoring the meat from the inside out so make sure you season your brine with flavors you want in your turkey.
This makes enough brine for up to a 20 pound turkey.