To the lay man, bone marrow is simply the part of the bone that produces more blood, but academics and medicinal practitioners know that not all bone marrow produces new blood cells since the only one capable of doing so is the red marrow.
There are two basic kinds of bone marrow the first being the red marrow which will then later on develop into the second kind which is the yellow marrow. Red marrow is what everybody has and is tasked with the production of new blood cells through a process referred to as haematopoiesis. As a person grows and develops this same red marrow will later on change into the second kind of marrow, the yellow marrow. The yellow marrow will then no longer produce new blood cells but will then be used as a reserved storage space of fatty acids. Adults will usually only have red marrow in their ribs, vertebrae, femur pelvic bones.
The bone marrow, whether red marrow or yellow marrow, can be found in the inner cavity of the bone, this cavity is called trabecular bone. The trebecular bone will usually weight only a fifth of a bones total mass but will take almost 1000 percent more space then the surrounding compact bone. Something else to take note of is that the trabecular bone has a very very porous structure mostly due to the many paltes and rods that make-up its interior, imagine an empty building and that would be a good representation.