We can't see the electrons, but we see the remnants of them after they hit the screen and cause it to glow. So far all of the particles mentioned have mass. The photon is the quantum unit of electromagnetic energy, or the basic unit of light. Light, which is just electromagnetic energy, is found in discrete lumps, or quanta, of energy. This is the underlying feature in Modern Quantum Theory.
Light exhibits both particle and wave characteristics.
However, when we talk about light as a particle, we are talking about the massless photon, which always travels at the speed of light. You have seen that two like-poles of a magnet, when placed near each other will repel. Each magnet is exerting a 'magnetic force' on the other. Think of two people throwing a heavy object back and forth. The force is mediated through space by an exchange particle, or a virtual particle.