1. Energy exists in different forms and is called different names depending upon whether the energy is related to motion, position, vibration, sound, heat/thermal energy, etc. (though at the microscopic level, vibration, sound and heat/thermal energy are just motion of molecules/atoms). 2. Energy is conserved (conservation of energy), meaning that the total amount of energy in a closed system (no energy in or out) is constant. In other words, conservation of energy means that energy is not lost, it merely changes forms (kinetic energy, potential energy, heat/vibrations due to friction)