Plaque is the sticky, colorless film that constantly forms on your teeth. Bacteria live in plaque and secrete acids that cause tooth decay and irritate gum tissue. This irritation causes an inflammatory reaction by your body that can eventually lead to gingivitis and periodontal disease. If plaque is not removed regularly by tooth brushing and flossing, it hardens to create calculus (also known as tartar). Calculus cannot be removed with a toothbrush; only a dental professional can remove it during an oral cleaning. To keep plaque and calculus under control, it is essential to brush your teeth twice every day, floss at least once every day, and see your dental professional for regular cleanings.

From Perio.org