From the outside, a tooth looks like a hard, solid substance. But this cut-away illustration reveals that a tooth contains a lot more:

Enamel

The shiny, hard, white tissue covering the tooth is the strongest tissue in your body. It has to be! Your jaws place as much as 128 pounds of pressure on your teeth when you chew, bite, clench, or grind.

Dentin

This tissue makes up most of the body of the tooth. Even though dentin is hard and feels solid to the touch, it's actually microscopically porous (allows air / water to pass through it)  and needs a covering of enamel or an artificial crown to protect it from decay-causing bacteria in your saliva (spit).

Pulp

This soft tissue contains blood vessels, nerves, and connective tissue. The pulp provides nourishment for the tooth during growth and development.

Crown

This is the part of the tooth you can see above the gum line.

Root

This part of the tooth sits in the bone below the gum. Believe it or not, the root of your tooth is usually twice as long as the crown, the part you see above the gum line.

Bone

The roots of your teeth are anchored by bone. Healthy teeth stimulate and keep bone tissue healthy and vice versa.

Gum

Dentists call this the "gingiva." It covers the bone surrounding your teeth. When you brush your teeth after meals and floss daily, you keep this tissue healthy. That's important, because gum disease can cause bone loss. Gum disease can also expose the tooth roots to decay.