Cavities are caused by PLAQUE. Plaque is formed by a certain type of bacteria in the mouth that attaches itself to hard surfaces, like the enamel on your teeth. If they aren't removed, they grow and form colonies. Other bacteria of different types attach themselves to the colony already formed on your teeth. Proteins in your saliva also mix with the colony and form a whitish film on your teeth.
Sugar plays a harmful role in tooth decay. The bacteria that forms plaque use sugar as a form of energy. This causes the bacteria to grow faster and the plaque grows larger and thicker. Some of the sugar can be turned into a kind of glue that makes it harder to be washed off the tooth by your saliva.
The plaque produces acids that dissolve the minerals that make your tooth enamel hard. Tiny holes appear, then the acid causes the tiny holes to get larger a cavity.