What a photon is, no one knows. To explain the photoelectric effect, Einstein assumed that this thing should be much smaller than an atom. It should be just an energy body, and its energy is equal to the product of Planck constant and its frequency. It is a particle, but if it has mass, its mass should be close to infinity. The second problem is that the difference of light lies in its frequency. Visible light is only a small part of the electromagnetic spectrum. If a beam of light has the same frequency, it cannot be said that it is composed of many colors. However, due to different wavelengths and frequencies, different lights do have different functions and energies. By far, gamma rays are the strongest light.