concept of energy

Reduced to simplest terms, the universe is nothing more than a series of energy exchanges. As in much of physics, the concept of energy is easy to understand, but difficult to express and work with. 

Energy = The ability to cause change.

Notice the simplicity of this definition. If something has the ability to cause a change in the physical or chemical properties of another object it has energy. If we restrict our study to mechanical energy, then we consider the changes in the state of motion of an object. 

There are different classifications of energy, usually named for the source of the force involved; mechanical, gravitational, electrical, nuclear, etc. Within each of these areas the energy is further classified as either potential or kinetic energy. 
Potential energy - Energy due to an object’s position.