Traditionally personal computers have used the physics of magnetism as the primary means of storing programs and data for later use. Floppy disks, hard disks, Zip disks, and backup tapes all work by magnetizing small areas on the surface of the disk. In recent years magnetism has been complemented by optics as a storage mechanism. High-intensity light sources such as lasers burn information into the disc surface. (Disks that employ magnetism are spelled with a "k" at the end; discs that use optics typically end with a "c".) An alphabet soup of optical technologies are available, including CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RAM, and DVD-RW, with even more on the way. You might think that making sense of these acronyms requires a Ph.D. Not necessarily, though it can be confusing. Here's a rundown. CD-ROM -- This was the first popular optical disc technology used with PCs. CD-ROM stands for Compact Disc Read-Only Memory, which simply means your computer can read data from these discs but can't write data back. These discs hold 650 megabytes of data, a megabyte (MB) being approximately 1 million bytes, a byte being the equivalent of an alphabetical letter or a numeral. You can store 600,000 typewritten pages on one of these discs. Most computers today still come with CD-ROM drives -- drives are the mechanisms that spin the discs -- but this technology is being superseded. CD-R -- This is a newer technology that overcame CD-ROM's read-only limitation -- CD-R stands for Compact Disc Recordable. Popular uses are copying music and archiving data. The best feature of these discs is their low cost -- 20 cents in bulk. The biggest limitation is that you can record data onto them only once. CD-R drives can read both CD-R and CD-ROM discs. CD-RW -- Standing for Compact Disc Rewritable, CD-RW overcame the write-once limitation of CD-R. You can rewrite and erase data multiple times. It's a technology that's becoming increasingly popular in new PCs, in many cases replacing Zip and tape backup drives. CD-RW now has a cost advantage over Zip for backing up data or moving it from one PC to another. CD-RW drives cost twice that of Zip drives, but the discs themselves are 10 times less expensive than the Zip discs and have two times more capacity. The latest CD-RW drives are as fast in reading data as Zip drives and nearly as fast in writing data. They're faster than tape backup drives and more versatile. CD-RW drives can write to CD-R or CD-RW discs. DVD-ROM -- This is a technology that promised much but never quite lived up to its potential. Standing for Digital Versatile Disk Read-Only Memory, DVD-ROM uses discs that are similar to CD-ROM discs but typically hold seven to eight times more data. Despite the greater capacity of DVD-ROMs, software makers have continued to distribute their programs primarily on CD-ROMs because of the ubiquity of CD-ROM drives. DVD-ROM drives can be useful for watching DVD movies on your PC or playing computer games. DVD-ROM drives, like CD-ROM drives, can't record data, though they can read most types of CD and DVD discs. The speed ratings of DVD-ROM drives aren't comparable with those of CD-ROM drives -- a 12X DVD-ROM drive is faster than a 48X CD-ROM drive. DVD-R -- Similar to CD-R, this technology lets you record data onto discs, but only once. DVD-R discs currently have seven to eight times the capacity of CD-R discs, though both the drives and the discs are more expensive, with the discs costing about $12 each. DVD-R drives can create CD-R discs and create or rewrite CD-RW discs. DVD-RAM -- This is a competing, and incompatible, technology. You can't read DVD-RAM discs with most other DVD drives, and DVD-RAM drives can't create discs that can be read by CD-ROM drives or CD players, a big limitation compared with DVD-R. DVD-RW -- This technology lets you record, erase, and rerecord data. Because it's a new technology, it's expensive, with the discs costing $20 to $35 each. Some DVD-ROM drives can't read DVD-RW discs that have been written to multiple times. Of the above technologies, CD-RW and DVD-ROM are versatile, compatible, and cost-effective choices. Regarding the newer DVD technologies, "it's impossible to call a winner," says Mary Craig, optical storage analyst for Gartner Dataquest, a market research firm in San Jose. Unless you need to buy immediately, your best bet is to wait to see which technologies are adopted over time by the major computer makers. Reid Goldsborough is a syndicated columnist and author of the book Straight Talk About the Information Superhighway. He can be reached at [email protected] or http://members.home.net/reidgold.