What is conservation of energy?

       The law of conservation of energy is an empirical law of physics. It states that the total amount of energy in an isolated system remains constant over time (is said to be conserved over time). A consequence of this law is that energy can neither be created nor destroyed, it can only be transformed from one state to another. The only thing that can happen to energy in a closed system is that it can change form, for instance chemical energy can become kinetic energy.

Definition provided by Wikipedia

Work and Energy  

The TRANSFORMATION of energy from one type (like Kinetic to potential) is reffered to as WORK.