Energy is a scalar physical quantity which is defined as the ability to do work. When a force applied on an object moves the object, work is said to be done and energy is spent. There are various types of energy present in the universe: kinetic energy, light energy, chemical energy, sound energy, electrical energy, mechanical energy, heat energy, elastic energy and gravitational energy.
The various types of energy are classified on the basis of the way the energy is apparent, the origin of energy and the way energy is carried. The total energy in a system doesn’t change with time but its frame of reference determines the value of different types of energy. For example, a passenger seating in a moving airplane does not have any kinetic energy relative to the plane, but has non zero kinetic energy comparative to earth. The various types of energy are interchangeable from one form to another. This is in accordance to the law of conversation of energy for an isolated system. The various types of energy can be transformed into matter and vice versa. The different types of energy are either renewable or non renewable. Renewable energy is that which can be renewed for future use. Non renewable energy can be just used once.
Energy is required to do all forms of activity. Every organism present on this planet depends on an external source to derive some type of energy to be able to do work. For example, plants need sunlight, animals need some form of chemical energy.