Cavities are permanent damage to the hardened outer layer of the tooth known as the Enamel. Bacteria in your mouth works night and day to recycle bits of leftover food and convert it into plaque. When plaque sits on your teeth is then digested by other forms of bacteria leaving behind an acidic mixture. These acids will eat away at your enamel, creating cavities.
When a cavity forms it will require treatment in the form of a filling or a crown. Smaller cavities are easily repaired with simple fillings. Larger cavities will often require a full out crown.
If a cavity is left untreated it can cause serious reduction to the structural integrity of the tooth. The teeth then are more easily chipped and broken. Additionally, the cavity will move quickly through the softer inner layer of the tooth, called the Dentin, to reach the pulp.
If bacteria is allowed to reach the pulp layer it will likely require treatment in the form of a Root Canal.