A recorder encodes (or burns) data onto a recordable CD-R, DVD-R or DVD+R disc (called a blank) by selectively heating parts of an organic dye layer in the disc with a laser in its write head. This changes the reflectivity of the dye, thereby creating marks that can be read as with the "pits" and "lands" on pressed discs. The process is permanent and the media can be written to only once.

For rewriteable CD-RW, DVD-RW and DVD+RW media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc. Depending on the amount of power applied, the substance may be allowed to melt back into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created. Most rewriteable media is rated by manufacturers at up to 1000 write/erase cycles.

http://en.wikipedia.org/wiki/Optical_disc_recorder

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