Note that this question can also be asked at a competitor's website: Multivax. :-P
I surely do hope that we can collect sufficient data in here. ;-)
What is Entropy? It's actually one of the holy pillars of today's conventional science. It's basically a little fancier elaboration on the laws of thermodynamics which we surely have all learnt about in school. Entropy basically means that everything goes from order to chaos. Order basically stands for the energy and structure we need to survive.
For example, after we burnt a piece of wood, we cannot reconstruct that piece of wood from the smoke anymore. If we tried to catch all smoke particles in order to do so, we would have to invest a lot of effort and energy, in turn creating much more entropy and chaos than the order that we desperately tried to restore.
I have my own take on this but I guess I will formulate it into a new question the other day.
asked 06 Jan '10, 22:41
You can't reconstruct the burnt piece of wood, but you can grow a new tree.
When you grow a new tree, the tree takes energy from the sun and uses it to convert raw materials (carbon dioxide, water, etc.) into organized structures. This is how entropy is reversed (if only temporarily, and of course on a much smaller scale than in Asimov's story).
All living things create order from disorder by using energy to build ever more complex and intricate structures.