Tartar sticks to teeth of even healthy people who conscientiously brush their teeth after every meal. In technical lingo, this is called a calculus deposit, caused by the interaction of saliva and food debris on the surface of teeth.
The surface of tartar is porous, similar to a pumice stone, which permits the growth and propagation of bacteria. The bacteria gradually spreads to the gums, causing them to become inflamed and bleed.
This inflammation of the gums damages the bone which surrounds and supports the teeth. The bone then recedes, causing pockets between the teeth and gums, which then causes puss to form, loosening the teeth. This could lead to the loss of a tooth. A check-up at least once every six months is recommended to prevent periodontitis.