Here's a great site for all questions CD related:
From the FAQ:
"Subject: [1-1] What's CD-R? CD-RW?
CD-R is short for "CD-Recordable". Recordable CDs are WORM (Write Once, Read Multiple) media that work just like standard CDs. The advantage of CD-R over other types of optical media is that you can use the discs with a standard CD player. The disadvantage is that you can't reuse a disc.
A related technology called CD-Rewritable (CD-RW) allows you to erase discs and reuse them, but the CD-RW media doesn't work in all players. CD-Rewritable drives are able to write both CD-R and CD-RW discs.
All CD recorders can read CDs and CD-ROMs, just like a standard CD-ROM drive."
What the FAQ doesn't state in this particular section is that most burning software will allow multi-session disks. That is, you can write small bits of data until the disk is full (in the case of CD-Rs). You can have multi-session CD-RWs as well, but obviously, it doesn't *really* get full since you can overwrite data. A decent CD-RW will be fine up until about 1000 re-writes.