Osteoclasts are the bone breakdown cells. Osteoclasts dissolve the mineral in old bone and create pits or holes in the surface of the bone. This is called bone resorption. The bone building cells, osteoblasts, then line up and fill in the pits.
The process of bone breakdown and formation called bone turnover replaces the tissue of the skeleton many times during your life. Your entire skeleton is replaced about every 10 years.