Dental plaque is a clear/semi-clear material that allows bacteria to remain on the teeth. If dental plaque is not removed frequently (at least once a day) by proper brushing and flossing, the plaque becomes tightly attached to the tooth and only mechanical cleaning can remove it. This is why frequent visits to a dentist and regular, thorough cleanings by a dental hygienist are very important.
Plaque contains bacteria that combines with carbohydrates from your food producing acid which breaks down tooth structure. This process called demineralization will eventually cause decay to begin in the tooth. Once decay is present, it does not go away. It eats away the enamel more and more each day. The only way to stop the decay is to remove it and replace the void with a filling, onlay or crown, depending on the amount of decay present.