That sticky, invisible biofilm (a colony of microorganisms) that forms daily on your teeth is called plaque. If it's allowed to, harden, it's called tartar or calculus. Plaque contains bacteria, which turn sugars and starches from food into an acid that causes tooth demineralization (dissolving of the enamel) and leads to tooth decay (cavities, or caries). Calculus irritates the gums, which brings about gingivitis (inflammation); if left untreated, this can develop into the more serious periodontal disease.
The bacteria that live in the human mouth are not normally
harmful, but if they are not
removed by brushing, they can build up into a thin layer. Then the bacteria nearest
the tooth surface
begin to metabolize
food with anaerobic respiration (without oxygen); the waste ducts of that
process are very acidic.
Daily brushing, flossing, and use of mouthwash are the
primary preventative methods. Dentists can
remove plaque in a process called scaling, if gum disease is present, root planting is utilized. (All Facts Considered, by Kee Malesky)