Entropy: An Introduction
A special thanks to:
Frank L. Lambert, Professor Emeritus (Chemistry)
Occidental College, Los Angeles
for his permission to reproduce this work.

The second law of thermodynamics is a powerful aid to help us understand why the world works as it does -- why hot pans cool down, why our bodies stay warm even in the cold, why gasoline makes engines run.  Entropy also is simple to describe and explain qualitatively.  However, to begin our qualitative approach we must avoid the briar patches involving the second law and entropy that have been planted all over acres of book pages and Web sites.
For those who prefer conclusions before explanations:
The second law of thermodynamics says that energy of all kinds in our material world disperses or dissipates if it is not hindered from doing so. Entropy is the quantitative measure of that kind of spontaneous process: how much energy has flowed from being constricted or concentrated to being more widely spread out (at the temperature in the process).
From the 1860s until now, in physics and chemistry (the two sciences originating and most extensively using the concept) entropy has applied only to situations involving energy flow that can be measured as "heat" change, as is indicated by the two-word description, thermodynamic ("heat action or flow") entropy. 
Entropy is not disorder, not a measure of chaos, not a driving force. Energy's diffusion or dispersal to more microstates is the driving force in chemistry. Entropy is the measure or index of that dispersal. In thermodynamics, the entropy of a substance increases when it is warmed because more thermal energy has been dispersed within it from the warmer surroundings. In contrast, when ideal gases or liquids are allowed to expand or to mix in a larger volume, the entropy increase is due to a greater dispersion of their original unchanged thermal energy. From a molecular viewpoint all such entropy increases involve the dispersal of energy over a greater number, or a more readily accessible set, of microstates. 
For several years students were taught that "Entropy is disorder," Entropy is NOT disorder! This confusion about disorder and entropy comes from 1895 before an adequate understanding of the details of energy change in atoms and molecules was possible. At that time even the existence of molecules was not acknowledged by some of the most prominent scientists in physics and chemistry. Those who proposed and elaborated the second law had no better catchall phrase to describe to others what they believed was happening. Order/disorder became increasingly obsolete to apply to entropy and the second law when the existence of quantized energy levels in physics and chemistry began to be understood in the early twentieth century.
Although order/disorder is still present in some elementary chemistry texts as a gimmick for guessing about entropy changes, it is both misleading and an anachronism today and will be phased out of future textbooks. In the humanities and popular literature, the repeated use of entropy in connection with "disorder" (in the multitude of its different common meanings) has caused enormous intellectual harm. Entropy has been thereby dissociated from the quintessential connection with its atomic/molecular energetic foundation. The result is that a nineteenth century error about entropy's meaning has been generally and mistakenly applied to disorderly parties, dysfunctional personal lives, and even disruptions in international events. This may make pages of metaphor but it is totally unrelated to thermodynamic entropy in physico-chemical science. It is as ridiculous as talking about how Einstein's relativity theory can be applied to a person's undesirable relatives in Chicago.