Conservation of Energy
Conservation of Energy is a principle of physics that the total amount of energy in a closed system remains constant. Energy is often converted from one form into another, such as the conversion of potential energy into kinetic energy when a ball is dropped, but the overall amount of energy remains unchanged.
Heat was a latecomer to this game. The field of Thermodynamics, developed during the 19th century by such people as Carnot, Gibbs, Clausius, Clapeyron, Maxwell, Helmholtz, and Thompson (Lord Kelvin), established that, among other things, heat was a type of kinetic energy. The First Law of Thermodynamics is simply a statement of that fact. Because the connection between heat and more conventional forms of kinetic energy involves esoteric issues of statistical mechanics, thermodynamics is actually a rather advanced subject. The Second and Third laws of thermodynamics deal with these issues.