Tooth decay is another name for the disease known as "caries", or cavities. A cavity is the result of your tooth enamel, dentin, or cementum being destroyed over long-term exposure to harmful bacteria and other germs.
Cavities, while not life threatening and highly preventable, affect most people to some degree during their lifetime. Tooth decay is caused by your teeth being frequently exposed to foods rich in carbohydrates (starches and sugars) like soda pop, candy, ice cream, milk, and cakes. Ironically, even fruits, vegetables and juices can lead to tooth decay.
When these foods break down in your mouth, they release natural bacteria, which eventually turns into a colorless film on your teeth and gums called plaque. The plaque interacts with deposits left on your teeth from sugary and starchy foods to produce acids. These acids break down tooth enamel over time by dissolving, or demineralizing, the mineral structure of your teeth. This leads to tooth decay and weakening of the teeth.