Every event in the natural world results in an increase in entropy. Time is not symmetrical, it only moves in one direction, therefore entropy will always increase. One example of increasing entropy is water falling over a dam. When the water is above the dam it has some potential energy due to gravity, which can be used to generate electricity or turn a wheel to perform some useful task. This change in the energy state of the water as it falls over the dam is an increase in entropy.
Unavailable energy is most often in the form of thermal energy.
Thermal energy is the result of random and disorderly motions of huge numbers of individual atoms and molecules. Whereas kinetic energy, such as that of a moving object is the result of the orderly motion of all the atoms in the object. In order to harness thermal energy, two reservoirs of heat at different temperatures are required. The second law of thermodynamics tells us that the entropy of an isolated system tends to increase with time. An isolated system is one in which matter and energy cannot enter.