Bone marrow is the soft, jelly-like tissue that is found in the hollow centre of all large bones. It contains stem cells which produce red blood cells that carry oxygen around the body, white blood cells that fight infection, and platelets that help stop bleeding.
All the different types of blood cells that are produced by the stem cells are released into the blood stream through the veins and thin tissue surrounding the bone. Without bone marrow and stem cells, blood cannot be produced and we cannot survive.
Definition from NHS Choices: Bone marrow