The first law of thermodynamics is often called the law of the conservation of energy (actually mass-energy) because it says, in effect, that, when a system undergoes a process, the sum of all the energy transferred across the system boundary--either as heat or as work--is equal to the net change in the energy of the system. By convention, heat transfer to the system from its surroundings is usually taken as positive (heat transfer from the system therefore being negative), and work done by the system on its surroundings is generally considered positive (work done on the system being negative).
The second law of thermodynamics states that, in a closed system, the entropy does not decrease. That is, if the system is initially in a low-entropy (ordered) state, its condition will tend to slide spontaneously toward a state of maximum entropy (disorder). For example, if two blocks of metal at different temperatures are brought into thermal contact, the unbalanced temperature distribution (which represents a partial ordering of the energy) rapidly decays to a state of uniform temperature as energy flows from the hotter to the colder block. Having achieved this state, the system is in equilibrium.
The approach to equilibrium is therefore an irreversible process. The tendency toward equilibrium is so fundamental to physics that the second law is probably the most universal regulator of natural activity known to science.
An insight into entropy and the second law can be provided by the study of the microscopic constituents of matter (atoms and molecules). The temperature and pressure of a gas are traced to the agitation of the gas molecules. The entropic progress from order to disorder may then be viewed as an example of the general tendency of chaotic disruptions to disturb organization and structure. Any system that is subject to random agitations will eventually attain its most disordered condition.
The concept of temperature enters into thermodynamics as a precise mathematical quantity that relates heat to entropy. The interplay of these three quantities is further constrained by the third law of thermodynamics, which deals with the absolute zero of temperature and its theoretical unattainability. Absolute zero (approximately -273 C) would correspond to a condition in which a system had achieved its lowest energy state. The third law states that, as this minimum temperature is approached, the further extraction of energy becomes more and more difficult.
Excerpt from the Encyclopedia Britannica without permission.