Energy is the ability of an object to do work. Two types of energy include potential energy and kinetic energy. Potential energy is the stored energy in a system due to its position. Kinetic energy is the energy of motion. Moving things have kinetic energy. The law of conservation of energy states that energy can be transferred from one type to another but is always conserved. Potential energy can be transferred into kinetic energy and kinetic energy can be transferred into potential energy but the amount of energy remains the same even though it may be in a different form.
Example: A roller coaster sitting on the top of a hill has potential energy. When it starts to move down the hill it has kinetic energy. Energy was transferred from potential to kinetic but the amount of energy remains the same.