Bone cancer is caused when cancer cells form in the bones and creates malignant growths, which can be fatal.
Of note, primary bone cancer (e.g. osteosarcoma) is relatively uncommon. Most people who are reported as having "bone cancer" actually have another cancer that has "metastasized" or spread to the bone. For example, lung cancer commonly metastasizes, or spreads, to the bones; However, the patient would not be considered to have "bone cancer" and "lung cancer". It would be considered metastatic lung cancer or lung cancer with metastases to the bones.