A Compact Disc Rewritable, or CD-RW, is a rewritable version of CD-ROM. Whereas standard prerecorded compact discs have their information permanently stamped into an aluminium reflecting layer, CD-RW discs have a phase-change recording layer and an additional aluminium reflecting layer. A laser beam can melt crystals in the recording layer into a non-crystalline amorphous phase, or anneal them slowly at a lower temperature back to the crystalline state. The different reflectance of the resulting areas make them appear like the 'pits' and 'lands' of a standard CD.
A CD-RW drive can write about 700MiB of data to CD-RW media around 1000 times. The number of times the CD-RW can be re-written varies depending on the quality and production techniques employed. Most CD-RW drives can also write once to CD-R media. Except for the ability to completely erase a disc, CD-RWs act very much like CD-Rs and are subject to the same restrictions; i.e., they can be extended, but not selectively overwritten, and must be closed before they can be read in a normal CD-ROM drive. A variation of UDF formatting allows CD-RWs to be randomly read and written, but limits the capacity to about 500MB.