Plaque
Research has shown that controlling plaque is important in the control of decay and gum disease. Plaque is neither food or food residue. Plaque is a clear, sticky deposit of bacteria that adheres to the surface of teeth and gum tissue. It is so adherent that it can only be removed by mechanical cleansing. Plaque contains a variety of different types of bacteria. For this reason, certain types of plaque are associated with dental decay, others with calculus formation, and others with the inflammatory response of the gums (gingivitis).
Plaque begins forming on the teeth in as little as 4 hours after brushing. This is why it is so important to brush your teeth at least twice a day and floss daily. The rate at which plaque forms and the location in which it develops can vary between individuals and even between different teeth in the same mouth. One of the prime areas in which plaque accumulates is at the gingival margin and sulcus where the tooth meets the gum.
Calculus
Plaque which is not removed regularly by brushing and flossing can harden into calculus (also called tartar). Calculus is plaque that has mineralized, forming a tough, crusty deposit that can only be removed by your dentist or hygienist. These deposits can form above (supragingival) and below (subgingival) the gum line. Calculus deposits are a significant contributing factor in periodontal disease because it is always covered by a layer of nonmineralized plaque. The calculus keeps the plaque close to the gingival tissue and makes it much more difficult to remove the plaque bacteria. Thorough removal of these deposits is necessary to prevent the progression of periodontal disease.