When you get "plaque" on your teeth, what you are getting is a "film of bacteria." These bacteria stick to your teeth, and one of the things they do is create acid as a by-product of their metabolism. The acid can eat through the enamel of the tooth and cause a cavity -- a hole through the enamel.
There are three parts to a tooth:
- The outer, very hard enamel
- The inner, much softer dentin (Dentin is like ivory.)
- The core pulp, consisting of nerves and blood vessels
Once the acid breaks through the enamel, bacteria invade the dentin and can eat through it fairly quickly. If they make it to the pulp, that is when you get a tooth infection and need a root canal.
When a dentist drills a tooth, what he or she is doing is shaping an opening. The first reason to do this is to ensure that all of the rot in the dentin is completely removed. The second reason is to create a shape to which the filling (either silver/mercury amalgam or plastic) can bond and fill completely.
Here are some interesting links: