CD-ROM
CD-ROM is actually a generic name for a family of optical disks (usually spelled "discs" in this case, for whatever reason) though it specifically refers only to a Compact Disc - Read-Only Memory. There are also CD-Rs (Recordable) and CD-RWs (Read-Write). CDs are much faster than floppy disks but slower than hard drives.
The rated capacity of a CD-ROM is usually 650 or 700 MB, but some CD drives are capable of "overburning", where the data track is spiraled more tightly than the CD specification. Not all drives will be able to read the resultant overburned disc, but this can push the capacity of a CD as high as 900 MB.
Other advantages of CDs over floppies are the near ubiquity on most current PCs of CD-ROM drives that can read data, plus the tendency of data to remain uncorrupted for much longer periods of time on CDs than for floppies.
The CD-ROM drive read speeds are described by a multiplier. A 1x drive is capable of transferring 150 kb/s, so a 24x drive can read at rate of 3600 kb/s. These values are however only theoretical maximum values, which are reached only when reading data from the outermost edge of the disc.
A CD-ROM drive uses an infrared-wavelength laser to read the pits and grooves in the surface of the disc. These features are only about 1.5 microns wide; that is, a little more than a millionth of a meter. Dust particles are typically much bigger than this. The CD gets around this limitation by using a particular grade of optical plastic over the metallic data surface. When the laser beam reaches the plastic, it is about a millimeter wide, bigger than dust, preventing dust from entirely occluding it. When it enters the plastic, the difference in the refractive index between air and the plastic causes the beam diameter to shrink; by the time it reaches the data surface of the disc, it is the approximate size of the features it is reading.