Poaching is an incredibly versatile cooking method; just about everything from fruits to meats can be cooked using this technique. Poaching is merely simmering food in liquid until it is cooked through.
As with baking, the density of the food will determine the cooking duration time; fish is cooked for a short amount of time in a liquid that is gradually heated, while denser meats cook longer starting with a cold liquid, so as ensure thorough cooking.
The key to poaching meats and proteins is to make sure that your stove temperature is not too high and that your liquid does not come to a boil, as this will cause the meat to break down, resulting in a greasy meal in which the fats are no longer separated on the top of the liquid. (Most cooks choose to skim the fat off of the top of the poaching liquid when cooking meat this way, either reserving it for use in a gravy or sauce or simply discarding it.) Because eggs cook quickly, the liquid is first brought to a boil and then turned off. Then, the eggs are added and covered until cooked to the desired doneness. When poaching eggs, in order to keep the whites intact, it helps to add a bit of vinegar to the water (about 1 t. to 2-3 cups of liquid).