Mr Case showed us some of the components on the inside like the cd or half the cd better said. Cds work by having little holes in it and read by lasers . If the light comes back it's a 0 if th elight doesn't come back it's a 1 therefore it's using binary language. A cd has 56 bilion bits , a dvd has way more since it's 4.7 ot 9 gigabytes and a blu-ray disc can store incredibly more at around 25 gigabytes or 50 gigabites dual layer.These are called optical media.
A harddrive is similar to a cd but it's storage capacity is way more and uses magnets instead of pits . So any hard drive is like a big rewritable CD. But even rewritable cd's won't erase an write forever but hard drives will.