CD: the kind you buy from a store with content already on it. Mass-produced with methods and equipment not available in the consumer electronics market, because it was never really necessary. Also includes CD-ROM (Read Only Memory) for data/files read by a computer instead of music alone

CD-R (Recordable): can be written ("burned") once and only once. As mentioned in another comment, it may deteriorate over time because of how the disc gets written, but by the time that happens you'll probably forget you had that disc

CD-RW (ReWriteable): Can be written like a CD-R, but you can also erase it and write on it again. More expensive, and I believe some readers had trouble with it, but in a world where data storage was expensive and small this was still a useful thing to have

DVDs had a similar thing, except there were variants where the - was a +, eg DVD+R and DVD+RW. I can't remember the difference there, but it was pretty trivial. There was also a relatively obscure DVD-RAM that had random access memory. That was pretty cool as well, kind of an alternative to DVR that wasn't a VHS tape. No need to lose everything you had if you wanted to add more to it