There are different types. Most common and cheapest one people use is CDR which you basically cannot modify or rewrite over existing file. If you want one that you can format, buy the CDR-W type.
I put in a blank disc, and about to burn the cd with iTunes. I wasn't sure if I want to select audio cd or mp3 disc, so I hesitated and took the disk out. When I put it back in, I think it asked if I want to "prepare" it for something (I forgot what...maybe to format) and I click "enter". It started "prepare" which I cancelled. Now iTunes says it's not a blank disk (and it's not. I looked under explorer and it had 5MB of something on it). I then formatted it...hoping to wipe out everything..but it's still not blank. How can I make it blank again so that I can use it with iTunes.