The second law of thermodynamics states that the entropy, or disorder, of a closed system has a tendency to increase. Restoring order to an entropic system by separating the oxygen from the nitrogen in a room, for example, or driving water uphill, requires that more energy be added to the system than could be extracted from it.
The second law is statistical, and it results from the difference in the number of ordered states for any system relative to the practically infinite number of disordered states that system can exist in. Without the regular application of energy to drive the components of a system toward higher order, any system naturally drifts toward one of the essentially unlimited states of disorder until a state of equilibrium is reached.
Ever-increasing entropy is central to the human understanding of time. As time passes, systems generally become more disordered. One way to understand this is to picture the phases of water. As ice, water molecules are in an ordered state, organized as crystals, and thus have considerable entropic potential. As it is not permitted for heat to flow away from a cold body toward a warmer body, ice in a glass of water absorbs heat from its surroundings and melts. The molecules then become more entropic, or disordered. In the presence of heat, liquid water evaporates, becoming still more disordered in the process.