Stock: A strained liquid that results from simmering meats, fish, herbs, and vegetables in water. It is usually made by browning bones, vegetables and other ingredients before they're cooked in the liquid for hours. Used as a basis for soups or sauces.
Broth: A flavorful, aromatic liquid made by simmering water or stock with meat or vegetables.
Generally speaking, stock has a heartier, richer flavor. This is mostly due to the bones used (broth is made with meat only and no bones). In most recipes the two can be interchanged with similar results. Fine chefs might tell you different, but for regular home cooking, the dish will be fine. However, note that broth may end up saltier than stock, so you may need to adjust your recipe accordingly.