The Zero-Point Field (ZPF) is said to exist in a vacuum -- what is commonly thought of as empty space -- at a temperature of absolute zero (where all thermal radiation is absent; a condition obtained when reaching a temperature of absolute zero on the Kelvin scale). The background energy of the vacuum serves as the reference, or zero point, for all processes. Theoretical considerations indicate the ZPF should be a background sea of electromagnetic radiation that is both uniform and isotropic (the same in all directions).
The uniform and isotropic nature of the ZPF is important, and explains why it is not readily observed. Fundamentally, the lack of asymmetry of the ZPF prevents its easy identification, just as a fish being absolutely still in a sea of constant temperature and pressure water is unable to detect the water itself.
In some cases, motion through a medium can give rise to asymmetries, thus in turn allowing for the detection of the medium. However, in the case of the ZPF, motion through the “medium” (i.e. the field) at a constant velocity has not been shown to make the field detectable. This is because the field has the property of being "Lorentz invariant." (Lorentz invariance is a critical difference between the modern ZPF and nineteenth-century concepts of an ether.) In fact, the ZPF becomes detectable only when a body is accelerated through space.
There is, of course, a fundamental difference between “detectable” and “useable”. It is likely necessary to go beyond a simple, constant acceleration through space (in order to detect the ZPF), and instead, transition into a variable acceleration in order to tap into the energy of the ZPF. In this case, we can assume with a reasonable confidence that the greater the change in acceleration, the greater the energy derived from the ZPF.
Physicists Paul C. W. Davies and William G. Unruh, showed in the mid 1970s that a moving observer distorts the ZPF spectrum by accelerating through the field. Furthermore, this distortion increases with increasing acceleration. Extending these findings would suggest highly variable accelerations could provide increased distortions, and that these distortions could be used as an energy source. While these distortions are small, they add up rapidly. At the same time, detailed analysis shows that the distortions are fundamentally the origin of inertia.
In this regard, it has been shown that when an electromagnetically interacting particle is accelerated through the ZPF, a force is exerted on the charge. Furthermore, the force is proportional to the acceleration, but acts in the direction opposite to it. I.e., the charge experiences an electromagnetic force as resistance to acceleration. Which is the equivalent of the inertia of a massive particle, what Sir Isaac Newton regarded as an innate property of matter. Importantly, this allows for the idea that Newton’s Second Law (i.e. F = ma) can be derived from the laws of electrodynamics, provided one assumes a ZPF.
Additionally, it is no longer necessary to assume a physical quantity known as Mass, which has the property of inertia, in order to explain a resistance to acceleration. What is seen as inertia is nothing more than an effect caused by an electromagnetic force acting on a charge. In effect, charge and its interaction with the ZPF creates what we experience as mass. Mass may, in fact, be an illusion.
If the ZPF gives rise to the phenomenon of inertia, does it in some way generate the effect of gravity? Andrei D. Sakharov suggested as much in 1968, an idea which was addressed 20 years later by Puthoff. Using stochastic electrodynamics, Puthoff showed that if a charged particle is subjected to ZPF interactions, it fluctuates, simultaneously causing charged particles everywhere in the universe to also fluctuate. These fluctuations result in electromagnetic fields, which have an attractive force between them. This force is much weaker than the electromagnetic attractive or repulsive forces between electric charges. It is also always an attractive force, which suggests it is simply gravity. 
The fluctuations are relativistic -- i.e. moving at velocities at or close to the speed of light. The energy associated with the fluctuations can then be interpreted as the energy equivalent of gravitational rest mass. Since the gravitational force is caused by these fluctuations, physics no longer needs the concept of a gravitational mass as the source of gravitation. Instead, the source of gravitation is based on electric charge in motion.
The ZPF can be thought of as a sea of radiation that fills the entire universe. It involves highly energetic emissions, with the Zero-Point Energy density rising proportional to the cube of the frequency of the radiation. This implies that by doubling the frequency, the energy increases by a factor of eight.
Because the energy density of the ZPF increases as the cube of the frequency, the amount of energy making up the ZPF is enormous. That energy, in the conventional view, is forced into existence by the laws of quantum mechanics. It is regarded in quantum fashion as sometimes real and sometimes virtual, depending on the problem at hand.
A competing theory with respect to the ZPF comes from an obscure discipline within physics known as stochastic electrodynamics, which postulates that the ZPF is as real as any other radiation field, in fact, as fundamental as the existence of the universe itself. The only difference between stochastic electrodynamics and ordinary classical physics is the single assumption of the presence of this all-pervasive, real ZPF, which happens to be an intrinsic part of the universe.
This single assumption is justified in part because much of quantum phenomena can be derived by adding the ZPF to classical physics. Furthermore, this can be done without invoking the usual laws or logic of quantum mechanics. This suggests the option of either accepting the laws of classical physics as only partly true, with the necessity of adding a set of quantum laws to complete the picture -- something currently done in physics today. Alternatively, one could accept the laws of classical physics as the only necessary laws, but merely supplemented by the presence of the ZPF.