Well, internal energy is the collective energy of all the different types of molecules found within a system. Now, a system can be composed of billions of different molecules. For simplification purposes, we're going to look at a system with only two diatomic oxygen molecules, moles. Now, let's look at the types of energies molecules could have. Well, they can have either kinetic energy or potential energy. Kinetic energy is the energy due to motion, and it could be subdivided into three different types.