A bone scan is a study that looks at relative blood flow to parts of the body. A small amount of a patient's blood is withdrawn, and a short acting radioactive isotope is attached to the red blood cells. The 'radioactive' blood is then reinjected back into the body. The patient lays on a table, which is really a big geiger counter. Wherever the blood flows in the body will show up on the scan. It just so happens that the skeleton has a tremendous amount of blood flow through it, compared to the rest of the body. Therefore, a bone scan will provide a picture of blood flow through the skeleton. Certain conditions cause a relative increase in local blood flow, such as a fracture, infection, or tumor. A bone scan is very sensitive at detecting this. By the way, the radioactive isotope is very weak, short-acting, and not very dangerous. It is expensive, however.
CT scans (CAT scans, Computed Axial Tomography) are another form of specialized x-rays. They look a lot like a MR scan, and you also have to lie still on a table with the CT scanner over you. The CT scan will also provide detailed pictures of slices of your body, but it works best on bones, and works relatively poorly for soft tissues. The CT scan is useful to look for fractures or other bony injuries, when the plain x-ray is difficult to interpret.