A toothache occurs when the sugar bugs harm the tooth all the way into the pulp. As you can see on the picture, it would be a very deep cavity. A filling could fix it and the dentist might have to give you some medicine. But sometimes the dentist may have to remove the tooth because it is too sick. On occasion, a tooth can hurt if the sugar bugs only harm the dentin and because it isn't as deep, a filling would fix it.

What you see in your mouth is above the gum and the two roots below the gum is hidden in your bone.

Enamel is the white part of your tooth which you can see in your mouth. It is the hard part of the tooth.

Dentin is the yellow part of your tooth just under the enamel. It is softer and spongier than the enamel. The enamel and the dentin together help you bite and chew your food. 

The pulp is red because it has the blood and the nerves which will keep the tooth alive.

The gum is pink and raps around your teeth. The bone and the gum hold the tooth in your mouth.