Q: How is it stored on them?
A: On CD-ROMs from the manufacturer, they are pressed. On CD-Rs there is a dye in the cd. When a laser is focused properly on the dye it changes it. Both of these are read by a laser which picks up reflections from the surface. This data (which is digital, 1s and 0s) is sent to a controller which communicates this with the computer. I think this is as basic as I can get.