By nature, all reactions either release or absorb energy. Energy is the ability to do work, and can be measured in terms of the amount work that the energy can do or the amount of heat that it can become. The SI until for measuring energy is the joule, symbol J, which is equal to one newton-meter. The Law of Conservation of Energy states that the total amount of energy remains the same during all energy changes, and energy can neither be created nor destroyed as a result of changes in its form.
Work is the result of a force acting on a body and producing motion. Work is a product of the force exerted and the distance it acts across, work=force*distance.
Einstein discovered tht mass and energy are related. When a body or system gives off energy the body or system decreases in mass, and when a body or system gains energy it increases in mass. These changes in mass, however, are so small that they are negligable.