Plaque is a sticky, colorless film of bacteria that constantly forms on our teeth and along the gum line. Plaque contains bacteria that cause cavities and gum disease. As plaque forms and is not removed by proper brushing and flossing, it can harden into tartar — sometimes called calculus — which is calcified (or hardened) plaque that attaches to the enamel on your teeth, as well as below the gum line.