Energy:
Energy is the capacity to do work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms. All forms of energy are associated with motion. Any given body has kinetic energy if it in motion. A tensioned device such as a bow or spring, though at rest, has the potential for creating motion; it contains potential energy because of its configuration. Similarly, nuclear energy is potential energy because it results from the configuration of subatomic particles in the nucleus of an atom.
The most commonly seen forms of energy that you find around yourself are: heat energy, light energy, sound energy and electrical energy.
Energy can be converted from one form to another in various ways. Usable mechanical or electrical energy is, for instance, produced by many kinds of devices, including fuel-burning heat engines, generators, batteries, fuel cells, etc..