Soft, fatty substance that is found in the center of an animal's leg bones, as well as spinal bones. It is considered a delicacy, especially in Europe. It can be cooked in the bone and removed afterwards, or it may be removed first and cooked separately. It can be baked or poached and then spread on toast, or used in soup to add body and flavor.
Source: Pabulum (there's an interesting marrow recipe there too)