Plaque is a biofilm constantly forming on your teeth' surface. Bacteria can grow in the plaque. These bacteria secrete acids that can damage your teeth' enamel, leading to tooth decay and the formation of cavities. The acids can also irritate your gums and make them vulnerable to further infection.
Tartar is hardened plaque that usually forms at the base of your teeth and can’t be removed by brushing or flossing. It can cause gum irritation, and it can also host decay-causing bacteria. Tartar is removed by a dental hygienist or dentist using a special tool called a scaler.