Principle of conservation of energy :

The principle of conservation of energy states that , "Energy can neither be created nor be destroyed but may be converted from one form to another form." 

It is the fundamental law of Physics. For e.g.

1. When a speeding car hits a wall, the car comes to rest. Here, the kinetic energy of car is not destroyed but is converted into heat energy, sound energy. It will be seen that kinetic energy of car before hitting is equal to produced heat or sound energy after hitting. This shows that total energy is the same though it has changed its form.

2. When a bullet hits a target, the kinetic energy of bullet is converted into sound energy. kinetic energy of bullet is exactly equal to sound energy produced after hitting.