A CD, such as the kind you play to listen to music, is an example of an optical disc. So is a "video disc" (properly called a laser disk), such as the kind you can rent at the video store that has an entire movie on it. Optical discs for your computer can hold an incredible amount of information- up to 6,000 megabytes (which is 6 gigabytes) of data. Entire encyclopaedias, Shakespeare's works, or representations of the art in the Louvre have been recorded onto optical discs.
So what makes a disc an optical disc? Just that the information is stored and read using light, rather than magnetism like a standard hard or floppy disk. (Why is it optical disc but hard disk? I give up.) There are three basic types of optical discs, and each one requires its own special kind of drive to use the disc. All three types are like floppy disks in that you pop them in and out of the drive as needed, giving you a potentially unlimited amount of storage. Here are the types:
CD-ROMS are the same size as the music CDs, and they store information in exactly the same way-in fact, you can playa music CD on your computer's CD-ROM drive. The information is permanently encoded on the disk and you cannot change it, but you can read it as many times as you like (ROM stands for read-only memory). CD-ROMS are great for publishing or distributing large amounts of information.
A WORM (write once, read many) disc lets you write to (put information on) the disc yourself, but only once. After that, a WORM disc works just like a CD-ROM. Since you get to decide what information goes on the disc, WORM systems are good for keeping archival copies of information you have to store permanently.
Erasable optical discs can be erased and recorded over many times, a technology that is in its infancy but one that may change the way we store data. The erasable optical drives now available are a lot slower than hard disks, and they're more expensive, but the disc itself can't crash like a hard disk. So they're used mainly for backing up hard disks. Actually, the current version of this technology is more properly called "magneto optical," since the information is still encoded on the discs magnetically. However, a laser beam assists in the process, and a laser is used to read the information from the disc.