This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
First and foremost: you do not need to format a disc unless you're using a
packet writing program like DirectCD. If you're running a program to create
a CD, chances are good that you don't need to format it. If you're using
"drive letter access", i.e. treating the CD-R or CD-RW like a big floppy
disk, then you do need to format it.
Simple rule of thumb: don't format it. Most software that needs a formatted
disc will format it for you as needed.
Formatting and erasing are different things. Formatting prepares a disc
for recording. On a CD-R it writes a few basic things, on CD-RW it may
write to most of the disc. The fixed-packet formatting that DirectCD does
for CD-RW discs takes about 50 minutes on a 2x-speed rewritable drive.
Erasing, which can only be done to CD-RW media, restores the disc to a
pristine state. If you want to erase a disc, use the software that came
with your CD-ReWritable drive. Somewhere in the army of applications and
mountain of menus is the command you're looking for.
The difference between "erase" and "quick erase" is that the former erases
the entire disc, while the latter just stomps on the Table of Contents
(TOC). It's like erasing the directory off of a floppy disk. The file
data is still there, but since there's nothing pointing to it, the disc
appears empty. (Some people have asked if it's possible to recover data
from a quick-erased disc. Acodisc can do this; see section (4-35).)
The difference between "format" and "fast format" (such as is offered on
the HP8100/Sony CRX100) is of a different nature. Both format the entire
disc, and both operate at the same speed, but the "fast" format allows you
to use the drive before formatting has completed. After a few minutes, you
are allowed to access the drive while the formatting process continues in
Incidentally, most conventional (pre-mastering) software will refuse to record
on a disc that has been formatted for packet writing. In some cases the
error message may be a confusing remark that insists the disc isn't writable.