Quantum Field Theory: What are virtual particles in layman's terms?

It should be stressed that virtual particles are not a different category of particles. Interactions between particles are calculated using Feynman diagrams such as this
To summarize: we only see the particles coming in and out of the diagram. We don't know what happens in between, and we approximate this using Feynman diagrams and the concept of virtual particles. This is called perturbation theory. Virtual particles have no other role other than in Feynman diagram calculations, and in principle they have no existence outside of the mathematical framework of perturbation theory. If you don't use perturbation theory, then you don't need virtual particles at all.
Also, as I said, for each interaction there are infinite possibilities for virtual particles on the internal line. So if they existed, all infinite possibilities must exist as once (in a quantum superposition). But since we cannot observe them and know whether they actually exist, the question of their existence is only of interest to philosophers, not to physicists.
Do artifacts of mathematical calculations actually exist in reality? This question cannot be answered using our current scientific knowledge, so physicists don't bother with it, they only care that these mathematical calculations give results that agree with experiment.
The same can be said, for example, for the fields of quantum field theory themselves: do they actually exists or are they merely a mathematical tool? No one knows, and no one cares, as long as quantum field theory provides accurate results and its predictions agree with experiment better than those of other theories.
Many physicists think that the concept of virtual particles should be avoided entirely, because it is misleading and confusing, to the general public and also to physics students. As far as physicists are concerned, virtual particles cannot be observed, and thus they do not "truly" exist. That's why they're called "virtual".
In CPH Theory has been shown photons are combination of positive and negative virtual photons. Photon is a very weak electric dipole that is consistent with the experience and these articles are asserted. In addition, this property of photon (very weak electric dipole) can describe the absorption and emission energy by charged particles.
The virtual particle existence is a very important question and should be clarify. If virtual particles are mathematical artifacts and do not exist, some phenomena are not accurately explained by virtual particles, as “non-real” particles. For examples, virtual particles should not be used in the description of vacuum and/or the black hole radiation (Hawking Radiation). In that case, the black hole radiation is not a real effect, it is just a theoretical concept.