In general, the word energy refers to a concept that can be paraphrased as "the potential for causing changes", and therefore one can say that energy is the cause of any change.
The most common definition of energy is the work that a certain force (gravitational, electromagnetic, etc) can do.
Due to a variety of forces, energy has many different forms (gravitational, electric, heat, etc.) that can be grouped into two major categories: kinetic energy and potential energy.
According to this definition, energy has the same units as work; a force applied through a distance.
The SI unit of energy, the joule, equals one newton applied through one meter, for example.
Energy has no direction in space, and is therefore considered a scalar quantity.
Energy is a conserved quantity, meaning that it cannot be created or destroyed, but only converted from one form into another.
Thus, the total energy of the universe always remains constant.
One form of energy can be readily transformed into another; for instance, a battery converts chemical energy into electrical energy.