First, some definitions from the American Heritage Dictionary:
pol·y·mer (pŏl'ə-mər)
n. Any of numerous natural and synthetic compounds of usually high molecular weight consisting of up to millions of repeated linked units, each a relatively light and simple molecule.
[Greek polumerēs, consisting of many parts : polu-, poly- + meros, part; see (s)mer-2 in Indo-European roots.]
clay (klā)
n. 1.
--1. A fine-grained, firm earthy material that is plastic when wet and hardens when heated, consisting primarily of hydrated silicates of aluminum and widely used in making bricks, tiles, and pottery.
--2. A hardening or nonhardening material having a consistency similar to clay and used for modeling.
2. Geology A sedimentary material with grains smaller than 0.002 millimeters in diameter.
3. Moist sticky earth; mud.
4. The human body as opposed to the spirit.
[Middle English clei, from Old English clæg.]
And here are more definitions for polymer and clay from dictionary.com.
So, to rephrase...polymer clay is a synthetic compound of "many parts" that is plastic when wet [raw, uncured] and hardens when heated.
At my website I offer the following description of polymer clay:
Polymer clay is a man-made substance containing polyvinylchloride (commonly called "PVC") together with a plasticizer. It is a modeling medium that, when well-conditioned and properly cured, produces a strong, long-lasting, durable product. It comes in many colors and differing textures, is easy to work with, and can be purchased in most craft stores. Unlike earthen clays, polymer clay cures at 275 degrees Fahrenheit, and does not require a kiln for firing. Polymer clay remains soft and pliable until baked so it is ideal for projects that may take more than one sitting to complete. Before curing, different techniques and substances can be applied-to and/or mixed-with it to produce different effects. Pictures can also be transferred to it. Once cured, it can be sanded, painted, drawn-on, carved, back-filled, polished, or varnished. Polymer clay is a phenomenal medium with virtually limitless possibilities for the experienced artist as well as the novice.
Below is a list of links that should get you started if you'd like to do further research on polymer clay:
Wikipedia's article on Polymer Clay
The Glass Attic's summary of Polymer Clay Characteristics
Polymer Clay Central's Polymer Clay FAQ
PolyForm's Polymer Clay Tips and Techniques
James Lehman's Polymer Clay FAQ and What is Polymer?
Sarajane Helm's Polymer Clay FAQ and Polymer Clay Glossary
The Polymer Clay Spot's Polymer Clay FAQ
Sue Heaser's Polymer Clay FAQ
Next up...The Do's and Don'ts of Polymer Clay.
