I use CD-R for backup.
I backup a folder say Monday, and I can backup the same folder (with a different name) to the same CD on Wednesday. And so on every few days.
I have a number of CD-R disks with multiple folders, each of which were added over a number of weeks.
I use Nero 6 to do this. I dont change any of the options, but I believe the ability to write to it more than once is called multisession.
What I CANT do is delete any data that is on the CD-R, but I can ADD more data.