When a bone is fractured, a bone has been cracked or broken-- it maybe a small crack, or could be a complete break with the bone coming out of an open wound. Bone is more than just a support system for our body-- it is also filled with live tissue that, among other things, creates blood cells and can regenerate bone damage. In any case, the best thing that medical practitioners can do is reset the bone into its original position. Once the bone is back in a normal position, and any broken off and dead tissue is cleared out, the natural process of healing can begin. Our bodies are amazingly good at repairing damage to them, and the process of bone repair is no less well-orchestrated by our body, requiring only healthy nutrition and ample rest.

Wikipedia has a good description of how bone heals, under the topic 'Bone Fracture'. I quoted part of it below:

"The natural process of healing a fracture starts when the injured bone and surrounding tissues bleed. The blood coagulates to form a blood clot situated between the broken fragments. Within a few days blood vessels grow into the jelly-like matrix of the blood clot. The new blood vessels bring white blood cells to the area, which gradually remove the non-viable material. The blood vessels also bring fibroblasts in the walls of the vessels and these multiply and produce collagen fibers. In this way the blood clot is replaced by a matrix of collagen. Collagen's rubbery consistency allows bone fragments to move only a small amount unless severe or persistent force is applied.

At this stage, some of the fibroblasts begin to lay down bone matrix (calcium hydroxyapatite) in the form of insoluble crystals. This mineralization of the collagen matrix stiffens it and transforms it into bone. In fact, bone is a mineralized collagen matrix; if the mineral is dissolved out of bone, it becomes rubbery. Healing bone callus is on average sufficiently mineralized to show up on X-ray within 6 weeks in adults and less in children. This initial "woven"bone does not have the strong mechanical properties of mature bone. Bya process of remodeling, the woven bone is replaced by mature"lamellar" bone. The whole process can take up to 18 months, but in adults the strength of the healing bone is usually 80% of normal by 3months after the injury."