The Compact Disk (CD) has probably taken over from the floppy disk as the most common removable storage media and associated drive. A CD-ROM drive or compatible now comes with every PC as standard similar to the way a floppy disk drive did. Most computer software is still distributed in this format.
When a CD is created the data is written (imprinted) onto a layer making tiny indentations in the media's surface. These are called pits whereas, the area that is not changed are called lands. The data written, i.e. the pits and lands are written in the form of a spiral from the centre of the disk outwards. This differs from hard disks where the data is written in concentric circles.
CD Drives are usually differentiated by their speed, e.g. x2, x4, etc. Modern CD Drives are x44 and x50 meaning that they are up to 44 and 50 times faster than the original single speed CD drive. A typical CD can hold up to 650MB of data, i.e. the equivalent of 400 1.44MB floppy disks.
Compact Disks are faster, more robust and are not be affected by magnetic fields as are floppy disks, however, care should be taken when using the media to avoid scratches or anything else happening to the surface. Sometimes when a CD is not working properly it has to do with there being some foreign body on the surface.
Next: Other CD Formats