There are basically two types of disk storage for computers: magnetic and optical. Magnetic storage is represented by the standard floppy and hard disks installed in most PC systems, where the data is recorded magnetically on rotating disks. Optical disc storage is similar to magnetic disk storage in basic operation, but it reads and records using light (optically) instead of magnetism. Although most magnetic disk storage is fully read- and write-capable many times over, many optical storage media are either read-only or write-once. Note the convention in which we refer to magnetic as disk and optical as disc. This is not a law or rule but seems to be followed by most in the industry.

Some media combine magnetic and optical techniques, using either an optical guidance system (called a laser servo) to position a magnetic read/write head or a laser to heat the disk so it can be written magnetically, thus polarizing areas of the track, which can then be read by a lower-powered laser, as in magneto-optical (MO) drives.

At one time, it was thought that optical storage would replace magnetic as the primary online storage medium. However, optical storage has proven to be much slower and far less dense than magnetic storage and is much more adaptable to removable-media designs. As such, optical storage is more often used for backup or archival storage purposes and as a mechanism by which programs or data can be loaded onto magnetic drives. Magnetic storage, being significantly faster and capable of holding much more information than optical media in the same amount of space, is more suited for direct online storage and most likely won't be replaced in that role by optical storage anytime soon.

The most promising development in the optical area is that rewritable DVD drives are becoming so inexpensive that they're starting to replace compact disc-rewritable (CD-RW) drives, providing a truly high-capacity optical medium on which to distribute or back up data. Virtually all systems today have an optical drive of some type, and most have support for both DVD-ROM and CD-RW.

Optical technology standards for computers can be divided into two major types:

• CD (CD-ROM, CD-R, CD-RW)

• DVD (DVD-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW, DVD+R)

Both CD and DVD storage devices are descended from popular entertainment standards; CD-based devices can also play music CDs, and DVD-based devices can play the same DVD videos you can purchase or rent. However, computer drives that can use these types of media also offer many additional features.

In the following sections, you will learn how CD and DVD drives and media are similar, how they differ from each other, and how they can be used to enhance your storage and playback options.