Most optical drives today are combo CD/DVD drives that support all the DVD and CD formats.  The speeds of the drives are rated by their DVD-ROM and CD-ROM transfer rates.  The original DVD drive (1x) transferred data at 1.39MB per second, sufficient for movies.  By doubling the spindle RPMs, the data transfer rate increased to 2.77MB/sec (2x) and so on.
Nine Times the CD-ROM Rating
Each DVD X is equivalent to roughly nine X's of CD speed because a DVD 1x is 1.39MB and a CD-ROM 1x is 150KB.  See 
DVD player and 
CD-ROM drives.
               DVD-ROM Data
    DVD-ROM    Transfer Rate
    X Rating   (MBytes/sec)
       1x       1.39
       2x       2.77
       2.4x     3.32
       4x       5.54
       6x       8.31
       8x      11.08
      10x      13.85
      12x      16.62
      16x      22.16
      18x      24.93
      20x      27.70
      22x      30.47
      24x      33.24
A Lot of X's
This is the label from a Memorex combo CD/DVD drive.  Each "X" number means X times 1.39MB for DVDs and X times 150KB for CDs.