The Big Ideas
Heat is a form of energy transfer. It can change the kinetic energy of a substance. For example, the average molecular kinetic energy of gas molecules is related to temperature. A heat engine turns a portion of the input heat (thermal energy) into mechanical work. A second portion of the input heat must be exhausted in order for the engine to have repetitive motion. Therefore, in a practical engine it is impossible for all the input heat to be converted to work.
Entropy is a measure of disorder, or the variety of ways in which a system can organize itself with the same total energy. The entropy of any isolated system always tends to disorder (i.e. entropy is always increasing). In the universe, the entropy of a subset (like evolution on Earth) can decrease (i.e. more order) but the total entropy of the universe is increasing (i.e. more disorder).
Thermodynamics is the study of heat engines. Any engine or power plant obeys the laws of thermodynamics. The first law of thermodynamics is a statement of conservation of energy. Total energy, including heat, is conserved in any process and in the complete cycle of a heat engine. The second law of thermodynamics as it applies to heat engines gives an absolute limit on the efficiency of any heat engine that goes through repetitious cycles.