Tooth decay is the course that results in a cavity.  This picture explains where the cavity comes from in the tooth

The blend of bacteria and food causes tooth decay.

A cavity forms when the acids cause more damage than the tooth can repair. The black spots are the decay 

 Eat food with high calcium  Brush your teeth well with a special type of toothbrush  Brush your tongue so you can have a good breath smell  Buy a new toothbrush every 3-4 months

The dentist drills the tooth where the cavity is formed then he cleans and disinfects the hole and finally he places the filling in the location of the cavity. A filling is an item that plugs the cavity hole and returns a tooth to its original shape after the dentist has removed the decay.