Same here. It's hard to explain. This is from my PC User mag:
CD Speeds
With CD-R/RW drivers, you'll see ratings such as 40x12x48, which indicates the maximum theoretical drive speed when burning CD-Rs, rewriting CD-RWs and reading CD-ROMs. The 'x' refers to multiples of 150Kbps, the speed of the original 1x CD-ROM drive.
DVD Speeds
The theoretical maximum DVD burn, rewrite or read speed of the DVD drive, with the 'x' referring to multiples of 1,250Kbps.