This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
DD-R and DD-RW are Sony standards for "double-density" recordable and
rewritable discs. The discs hold 1.3GB of data, and are relatively
inexpensive, but aren't compatible with current CD or DVD players. You
can only read the discs in a DD-R/DD-RW drive.
The recorders form a middle ground between CD-R and DVD-R in terms of storage
capacity and price, but the lack of compatibility reduces their usefulness.
On the bright side, the drives are expected to be able to record on CD-R
and CD-RW media.