What is the difference between osteosarcoma and osteochondroma?

Sarcs are malignant. Osteosarcoma is a cancer starting in bones, most commonly in children/young adults, while osteochondroma is a non-cancerous bony spur, most often occuring in teenagers. Osteochondromas usually need no treatment and are only followed, while osteosarcomas require agressive treatment, often including chemotherapy, radiation therapy and surgery; the prognosis has improved a great deal recently.
Cancer. Osteosarcoma is a malignant (cancerous) tumor of bone. Osteochondroma is a type of benign tumor, usually occurring in bones near joints. Though benign, osteochondromas can lead to disfigurement and pain and may require surgical removal.