The interaction of charges by exchange of photons can be clearly and accurately described in the mathematical language of quantum electrodynamics; nevertheless attempts to describe such phenomena in verbal terms can be very confusing. This arises because it is not possible to form a consistent detailed mental picture of the process, as can normally be done in classical physics. Despite these difficulties, an attempt is made to discuss some basic properties of photons from a physical point of view, with special reference to two distinct descriptions of the wave aspects of virtual photons. These are in terms of, firstly, plane waves for which ω2/c2≠k2, and secondly, evanescent waves for which w2/c2= k2, but the component of k along one of the axes is imaginary.