Cavities, as their name implies, are essentially holes in your teeth caused by decay. You can sometimes spot a cavity, as the area surrounding the hole generally looks dark brown or gray.
The main culprits to blame for your cavity are certain types of bacteria in your mouth. These bacteria are contained in plaque and they interact with the carbohydrates and sugars in your food creating an acidic environment that dissolves the protective enamel on the outer layer of your tooth. Once the acid succeeds in eroding the enamel, your tooth is exposed, leaving just the softer dentin layer, which will ultimately cause the formation of a cavity. At this point the decay process rapidly speeds up and spreads deeper into the tooth.