This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
"ML" is short for "MultiLevel". Devices and media constructed by Calimetrics
(http://www.calimetrics.com/) boast 3x the storage capacity and 3x the
recording speed of conventional CD-R and CD-RW media.
CD technology works by measuring the light reflected from the surface
of the disc. Traditional discs only have two levels ("pit" and "land"),
ML discs have more than one. By increasing the effective bit density of
the media, you can write 3x as much data in one revolution of the disc,
improving both the storage capacity and the recording speed.
The technology requires minor changes to existing hardware, and requires
discs optimized for ML recording. Discs written with ML devices will not
be compatible with existing CD players and CD-ROM drives. However, ML
recorders are expected to be able to record on CD-R/CD-RW media as well,
so ML support could be a low-cost bonus feature on new drives.
[ Announced in early 2002, this never really materialized as a consumer CD