When an electron moves out of its regular orbit around the nucleus of an atom to occupy another level on the outer side, it will come back to its previous level releasing the energy it had gained that made it leave its orbit in the first place. This energy released is made of photons and it presents itself as light. This was a very elementary description of where light comes from. For instance, you can observe the phenomenon when you pass a piece of metal on a grinder. You will see the sparks of atoms jumping out of the piece of metal where electrons give off the light as they go back to their original levels after they had been moved away.
Self-reinforcing and self-propagating electromagnetic waves. There are no photons — they are just an abstract construct that is used in the modelling tool that is quantum physics. People tend to have the bad habit of mistaking these things as being real particles though, despite single-photon double-slit interference experiments showing they aren't (since a particle can't go along two different paths and then interfere with itself, only waves do that).
Electromagnetic Radiation. Every part of EM spectrum is caused by a different process. Visible Light is caused by transition of electrons at higher electron orbits falling to lower orbits, closer to nucleus. The particular lines (specific frequencies) as the Spectroscope reveals, offers us a a great tool of immense value with which we can know the interior of nebulae, stars even the reflected light of planets satellites.
Well, there are two major categories of matter in this universe: baryonic matter (aka normal matter), and dark matter. Only the normal matter produces light. Dark matter is dark because it neither produces nor consumes light. Light just passes through dark matter. The only effect dark matter has on light is that it bends the path of light with its gravity. In normal matter, the production and consumption of light is mainly achieved through the process where electrons gain and lose energy. When they lose energy, that energy is sent out as a photon of light. If that photon is then captured again by another electron, then that other electron gains that energy. Electrons are responsible for the vast majority of electromagnetic radiation ranges, from the lowest energy radio waves, to the X-rays. For example, visible light is produced when electrons jump up and down by one energy step. UV light is produced when electrons jump up and down by two steps. X-rays are produced when electrons jump up and down by way more than 2 steps. Beyond X-rays there is another higher energy range of photons, called gamma rays. Gamma rays aren't produced by electrons. Instead they are produced right inside the nucleus of an atom, by the protons and neutrons. Gamma rays are produced when protons and neutrons start slamming into each other or breaking apart during nuclear fusion or fission reactions. This is the only category of light that doesn't get produced by electrons. Since protons and neutrons are thousands of times more massive than electrons, the energy of light they produce are also thousands of times more powerful than the light coming from electrons.
