The Most Misunderstood Concept in Physics - Entropy
https://www.youtube.com/watch?v=DxL2HoqLbyA
Entropy is not energy; entropy is how the energy in the universe is distributed. There is a constant amount of energy in the universe, but the way it is distributed is always changing. So entropy is a measure of the randomness/disorder of a system or the energy unavailable to do work.
Stumbled upon the video and got stuck watching it. Interesting stuff, at least the less technical first part of the video. The video gives a good example of the concept through the sun and how the sun's energy distributes on earth.
Ah yes, a better more scholarly explanation:
Entropy is a state function that is often erroneously referred to as the 'state of disorder' of a system. Qualitatively, entropy is simply a measure how much the energy of atoms and molecules become more spread out in a process and can be defined in terms of statistical probabilities of a system or in terms of the other thermodynamic quantities. Entropy is also the subject of the Second and Third laws of thermodynamics, which describe the changes in entropy of the universe with respect to the system and surroundings, and the entropy of substances, respectively.
