White stock : A clear, pale liquid made by simmering poultry, beef, or fish bones.
Brown stock : An amber liquid made by first browning/roasting poultry, beef, veal, or game bones.
Fumet : A highly flavored stock made with fish bones.
Court bouillon : An aromatic vegetable broth.
Glace : A reduced stock with a jelly-like consistency, made from brown stock, fish stock, or chicken stock.
Remouillage : A weak stock made from bones that have already been used in another preparation. At times it is used to replace water as the liquid used in a stock.
Bouillon : The liquid that results from simmering meats or vegetables; also referred to as broth.
I believe that stocks are an important part of cooking because it can give certain foods a lot of flavor. It can give the food(s) a good smell. It also is a good base for many sauces and soups.