Zero Point Modules are so named because they use zero point energy. Zero point energy is present in all quantum mechanical systems (i.e. everything that uses energy and converts it to entropy). Zero point energy is the lowest amount of energy that such a quantum mechanical system can have. It is also commonly referred to as vacuum energy and remains when all matter is removed from a region of space. Since by definition zero point energy can never be depleted (a region of space can never have less than that amount), it is theoretically an unlimited and free energy source.
The energy is said to arise from various particles that spontaneously emerge, exist for fractions of a second, and are then annihilated by anti-particles. At very small distances, this particle/anti-particle turbulence is known as quantum foam, and it is theorized to be a remnant of the birth of the universe carried over to this day. (Also partially explained in the hypothesis is that not all matter in the universe is evenly distributed). This variation in energy is somewhat analogous to observing common objects under very small scales. For example, ordinary glass under extreme magnification is porous. Marbles, if expanded to the size of the earth, would have more variation in radius than the Earth does.
Despite the two particles annihilating each other, a small amount of energy is detectable. Because of the immensely small scales of time and size, as well as the Heisenberg Uncertainty Principle, the existence and subsequent annihilation of these particles does not violate the conservation of energy. The individual particles are thought to be generated perhaps by photons interacting with the space.
As built by Stark Black, a ZPM is a container of a large region of vacuum subspace instead of normal space, so that its size is manageable. The ZPM draws power from the zero point energy, a tiny amount from each small bit of the enclosed subspace that adds up to unimaginable power in total. Theoretically the ZPM should never become depleted, but since all mechanical devices have imperfections, it is likely there is a miniscule amount of loss in the system that eventually leads to the last of the energy leaving the ZPM. There is also the issue of entropy eventually winding the system down.
The "Container" For lack of a better word ressembles an orange glowing rounded crystal. A Black energy converter or understanding of it's functions is required to use it.