The osteoblasts are cells that induce bone formation and the osteoclasts are cells that induce bone resorption.
Bone is dynamic tissue that is constantly being removed and replaced with new bone.
Normally there is a balance between bone formation and bone resorption. If there is an imbalance in this equilibrium between bone formation and resorption, there is either a decrease or increase in bone density. Osteoporosis is a specific term defining a state in which bone tissue is reduced, but mineralization of the organic matrix is normal. Osteoporosis has a variety of possible causes, and consequently manifests in a number of different forms. The basic distinction between forms of osteoporosis is between those types that are generalized or diffuse, involving the entire skeleton, and those that are localized to a single region or bone.