Decay in the enamel is where a cavity has begun to form. The enamel protects the soft inner layer of the tooth and the pulp. Taking action by filling the tooth (hyperlink to fillings) this early on will prevent the cavity progressing into something more painful and costly.
If the decay has reached the dentin, the dentist is still able to restore the tooth if the decay is not too close to the pulp. When decay has reached into the dentin the pain increases and its important to get this treated as soon as possible.
When decay reaches the pulp, the only option for your tooth is to start Root Canal. The pulp of the tooth gets infected and can cause severe pain to hot and cold as the nerve dies. Failure to take action during decay of the pulp can cause an infection at the root of the tooth (abscess) and can cause complications in the mouth and body.