Stands for "Compact Disc Re-Writable."
CD-RW is the rewritable compact disc format. A rewritable CD can be written to, erased, and re-written hundreds of times. One disc can store up to 700 MB of data or 80 minutes of digital audio. Rewritable discs may not play correctly in all CD players, as the rewritable layer on the underside of the disc is not as reflective as the pressed layer on a CD-ROM. Writing to a CD-RW disc requires a compatible disc drive that supports rewriting optical media.
Once data is written to a CD-RW disc, it is read-only and cannot be edited; in order to to make any changes to the files on a disc, it must be erased and re-written. This process makes the format a poor choice for storing files that need to be frequently modified. Instead, the rewritable discs are better suited for frequent data backups, although the format's small capacity compared to DVD±RW discs limits that use as well.
An optical disc drive that can write discs has three lasers—a low-power laser that reads data, a medium-power laser that erases rewritable discs, and a high-power laser that burns data to a disc. A CD-RW drive burns a series of spots onto a reflective layer of a phase change metal alloy on the underside of the disc. At first, this metal alloy exists in a crystalline form that lets light through. A high-power burning laser heats the alloy past its melting point, which then turns opaque as it cools. These opaque spots encode digital data that the reading laser in the drive reads and decodes. When it's time to erase a disc, the drive uses the medium-powered erase laser to heat the alloy layer to its crystallization point.