Stock is a flavorful liquid. It involves simmering bones and vegetables. It's application is a extraction of flavor from ingredients with a liquid base.
White Stock: A clear pale liquid that is made by simmering poultry, beef, or even fish bones.
Brown Stock: It is an amber liquid made by simmering poultry, beef, veal, and games bones which have been browned first.
Fumet: It is similar to fish stock, and it is highly flavored stock made with fish bones.
Court Bouillon: An organic vegetable broth used for poaching fish or vegetables.
Glace: A reduced stock with a jelly. It is made from brown stock, chicken stock, or fish stock.
Remouillage: A weak stock made from bones that have been used in preparation.
Bouillon: A liquid from simmering meats or vegetables. It is also referred to as a broth.
Jus: A lightly reduced stock used as a sauce for roasted meats.
Vegetable Stock: This is normally made from mirepoix, leeks, and turnips. Tomatoes, garlic, and seasoning is also added to flavor to darken the stock.
I think stock is an important part of cooking because it gives the food a good aroma, and color. It sets the foundation to build a flavor. Stocks give you the option of preparing quick dinner.