What is tooth decay?
In short, tooth decay is a location on a tooth where so much of the tooth's mineral content as been dissolved away that a defect (a hole or a "cavity") has developed.

Now, let's back up a few steps and start a discussion about tooth decay so this blurb of a description makes more sense.

Tooth anatomy as it relates to tooth decay.
First off, you need to think of a tooth in terms of being a hard calcified object. Yes, teeth do have nerves in their centers and this tissue is soft, but the surface of a tooth (where tooth decay begins) is formed from types of tissues that are very high in mineral content. These tissues are called enamel and dentin. Our mock up of a dental x-ray shown to the right illustrates where the dentin and enamel aspects of a tooth are located.

Enamel
The vast majority of the surface portion of a tooth that is visible is covered by enamel. You've probably heard that tooth enamel is the hardest tissue found in the human body. This is true. Enamel is over 95% mineral in composition. Most of this mineral is a compound called hydroxyapatite which, as you probably already know, has a high calcium content.

Dentin
You may be surprised to learn that teeth are not solid enamel. Only the portion of a tooth nature intended to lie at and above the gum line is covered by enamel. The bulk of a tooth, both its root and inner aspects, is composed of another calcified material called dentin. Dentin also contains the mineral hydroxyapatite, but to a lesser degree than enamel. Only about two thirds of the content of dentin is mineral so, relatively speaking, dentin is "softer" than enamel.
Page 2 and more about tooth decay formation.

Synonymous terms for tooth decay.
There are two terms that are commonly used to refer to tooth decay. The most common of these is the word "cavity", which no doubt simply refers to the hole which often forms as a result of the tooth decay process. Another term that can be used interchangeably with "tooth decay" is the word "caries". This is the term you will most frequently find used in dental literature. The word "caries" is derived from the Latin word for "rot", which seems to be a reasonably accurate description of the tooth decay process.

Has tooth decay always been a problem for mankind?
No doubt throughout the entire history of humankind there have always been at least some individuals who have suffered severely from the effects of tooth decay. Cavities first became pandemic (an epidemic spread over a wide geographic region) however with the establishment of sugar plantations in the 1700's in the "New World". Subsequently, tooth decay affected yet greater numbers of people with the widespread cultivation of the sugar beet in Europe in the 1800's.