Actually, today you don't have to finalize the CD, as most of the CD readers know how to read not finalized CDs. At the past, before we had burners everywhere the CD-Rom drives couldn't read data from not finalized CDs.

Anyway, if you finalize a CD you won't be able to add data to it. If you leave a CD open (don't finalize it) you can burn new session on it, means you can reburn new data. The data you've burned before will stay on the CD and take place, but you won't see it. You'll be able to do this until you're out of space.

When you are close enough to finish the place on your CD it's better you close it, so you won't have problem reading it on some drives.

As to reversability, when leaving a CD open, you will be able to finalize it (or add data, as mentioned). After you finalize you can't reopen it.