The ability to do work...
Energy may be thought of as a way to quantify the ability to get things done. The more energy you have available to you, the greater is your ability to have an impact on your surroundings. Energy exists in several forms, potential energy, kinetic energy, thermal energy, electromagnetic energy, nuclear energy and more. Energy in a sense is the inverse of
Potential energy is energy available due to the relative position of objects, and the forces they exert on one another. It is stored energy that is realized when the objects are allowed to move in response to those forces. Nuclear energy and electromagnetic energy are special forms of potential energy.
Kinetic energy is energy due to the relative motion of objects. When we release the objects that had potential energy they will reposition themselves so as to reduce their potential energy and in the process gain some speed relative to one another, converting some of the potential energy to kinetic energy.
Thermal energy is energy available due to the difference in temperature between objects. In accordance with the
of thermodynamics, when hot objects are brought into contact with cooler objects, the hot get cooler and the cool get hotter. If this bringing together of hot and cool objects is cleverly done, useful work may be accomplished. Examples are the steam engine, electric generating stations and so on. When kinetic energy is expended, some of it normally goes into raising the temperature of objects.
When relativity is taken into account, energy and mass are different manifestations of a single mass-energy.