As everyone knows, CD is the abbreviation of Compact Disc. the ROM part is short for Read-Only Memory. A CD-ROM is an optical disk with a capacity of anything up to 1000 megabytes (1GB), although the most common have around 700 megabytes of space or data on them. Audio CDs and computer data CDs share a technology which allows the computer to read both although an audio player cannot read the non-audio computer data. A CD-R can be recorded on only once. Once the data has been 'burned' to the CD it is there permanently and cannot be erased. A CD-RW compact disc is Re-Writable and can be recorded onto, erased and re-recorded on many times. Most of the older audio CD players can only read CD-R discs but some of the newer models allow CD-RWs to be played too. The CD starts life as a piece of injection-moulded clear plastic in the form of a thin, flat circle with a hole in the middle. A continuous spiral of miniscule impressions is made on one side of this flat object. This spiral, or data track, is about half a micron wide (One metre = 1,000,000 microns) and where the edges of the track meet, the thickness is just over one and a half microns. The data track runs from the inside edge of the disk to the outside. A thin layer of reflective aluminium is sprayed over the data track, re-creating a flat surface. A thin protective coating of acrylic is sprayed over that, creating the label side of the CD. When buying blank CDs to store data on, it is a good idea to hold a sample CD up to a source of light (a window for example) and see how easy it is to see the light through it. The more opaque the CD the thicker the coating. A CD player is also called a CD-ROM drive and is a device that can read information from a CD-ROM. An internal CD-ROM players fits in a bay in the computer casing whilst an external player has it's own casing and plugs into a port on the computer, usually a USB port.
The differences between one CD player and another are mainly in three areas:
a) read speed: a 24X player can access data from the CD at twenty-four times the speed of a single-speed player, for example.
b) access time: the average time it takes the drive to access a particular item of data
c) data transfer rate: how much data can be read and sent to the computer in a second. These days, manufacturers tend to put CD (or even DVD) 'burners' in new equipment, as the variation in cost between producing a recording device and a non-recording one has narrowed greatly in the past couple of years. | 
