Compact Disc Rewritable
Compact Disc Read-Write
CD-RW drives and media are currently (1999) more expensive than CD-R drives and media. CD-R is sometimes considered a better technology for archival purposes as the data cannot be accidentally modified or tampered with, and encourages better archival practices.
Standard prerecorded CDs have their information permanently stamped into an aluminium reflecting layer. CD-WR discs have a phase-change recording layer and an additional silver (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 areas make them appear as the 'pits' and 'lands' of a standard CD.