According to the principle of conservation of energy, energy can neither be created nor destroyed. It can only be transformed from one form to another. For example a car transforms the chemical energy in fuel to mechanical(kinetic) energy as it moves and some of the energy is lost as heat energy due to friction etc. Usually, questions in exams involve conversion of kinetic energy to gravitational energy and vice versa. If a ball falls from a height it's gravitational potential energy is converted to kinetic energy and so on.
