Essentially, energy is matter and vice-versa. Consider matter to be "frozen" energy, in a solid form. When some energy is added to the matter (actually quite a lot aka fusion and fission), the matter melts back into energy. All energy has the ability to do work, but it can exist in several states. Electromagnetic radiation (light and heat), consists of almost all of the free energy in the universe. Almost all processes give off some EM radiation at some frequency. All things in the universe have a tenancy towards a lower amount of energy, meaning that eventually no energy will be transmitted anymore due to all energy levels everywhere in the universe being the same. Other types of energy include kinetic (when things move) and potential (energy stored to do work later) Essentially, all energy in the universe could be turned into matter at 0K and all matter could be turned into massive amounts of energy