Total energy is neither created nor destroyed. This is true for all classical systems. Relativistic and nuclear phenomena replaces this law with conservation of mass-energy.

what he said but in simpler terms. the universe recycles on a mass scale. nothing is ever truly destroyed, it just breaks down and disperses for later use as something else.

1. Energy is always in some form (light, gravitational potential, chemical potential, magnetic potential, electric potential, kinetic, sound, compression, heat, etc.) 2. Energy can be converted from one form to another 3. When energy is converted from one form to another, some is always converted to heat at the same time 4. When energy is converted from one form to another, the total energy you started with (in form 1) equals the total energy you end up with (in form 2) plus the heat 5. There are other things that are conserved besides energy: momentum, angular momentum, charge, and a few other things that relate to nuclear physics (in other words, that's not the only conservation law) 6. In relativity, one man's energy can be another man's mass, so the total (mass+energy) is conserved 7. The law of conservation of energy holds everywhere we have looked (it was first noticed in experiments in which a raised weight fell and pulled a string which turned a paddle-wheel which heated a bucket of water) 8. Some cosmologists claim that energy is not conserved in cosmological phenomena such as redshifts resulting from the stretching of space