Using a very simplistic but rewarding approach, we shall define entropy as a measure of disorder. A system (such as a room or an energy form) is in a state of high entropy when its degree of disorder is high. As the order within a system increases, its entropy decreases.
For better or for worse, nature 'likes' chaos, disorder, high entropy... In fact, much of our life consists in fighting this disorder! So nature 'likes' heat more than it likes 'work'. (See energy conversion efficiency.) This can be explained in terms of probabilities. Disordered states are simply more likely to exist (or emerge) than ordered states. The spontaneous direction of change is from a less probable to a more probable state, as illustrated above.
An important consequence of this fact of life is that heat flows spontaneously from a place where it is hot to a place where it is cold. If you want to cool your room on a cold winter day, all you need to do is open the window. This is analogous to a downhill slide, which requires no expenditure of energy. If, on the other hand, you want to cool your room on a hot summer day, you have to buy yourself an air conditioner. The air conditioner will expend energy (and a considerable amount of it, as we shall see) to pump or transfer heat 'uphill', from a cold room to the hot atmosphere.
Imagine that these two compartments are two rooms in your house. One is hot, say 80 degrees Fahrenheit (represented by red molecules of air in the right compartment); the other one is cold, say 50 degrees (represented by green molecules of air in the left compartment). When you open the door, heat flows spontaneously from the hot room to the cold room. (The molecules of air mix. In collisions of 'hot' molecules with 'cold' molecules, transfer of energy takes place: the former lose some of their energy and become 'warm'; the latter gain energy and become 'warmer'.)
If the size of the rooms is approximately equal, the temperature in both rooms will eventually reach a warm 65 degrees (represented by yellow molecules of air in both compartments). This final state is a state of maximum entropy (as can be shown using probability theory).
Preventing heat from escaping our homes in winter is thus quite a challenge. (Fighting mother Nature is not an easy task!) Preventing heat from entering our homes in summer is also a major problem. (This will be an increasingly important problem if the greenhouse effect is for real.) The consequences of this law of nature will be explored in our discussion of residential comfort.