Energy is the ability to do work or cause change. When one object does work on another object, energy is transferred. The exchange of energy from one object to another means that work has been done.
Energy exists in two different states depending on whether or not an object is moving.
A moving object can do work when it strikes an object. The energy an object has due to motion is called kinetic energy.
When a car is traveling down a road it is an object with kinetic energy.
When an object isn't moving but still contains stored energy due to its position or shape.
When a book is held in the air it has the potential to do work. If the person holding the book lets go, the book will fall to the ground.