What's with the lights/colors/motion you see when you close your eyes?

When I was a little kid I used to close my eyes and rub them with my fists, because of all those pretty colors (and my mom would yell at me to stop.) I never knew exactly what was going on there.

Why are those colors there? What are they? If your were in a completely dark place would you eventually stop seeing them (or see more of them)? Are there any people who don’t see them? Do blind people experience this? Why do I see more of them when I’m high?