A CD recorder, CD writer or CD burner is a compact disc drive that can be used to produce discs readable in other CD-ROM drives and audio CD players. CD recorders not only help reduce the cost of purchasing CD's but they also allow you to create your own compilation CDs, making you the DJ. There are two different types of CD recorders: single-disc and dubbing CD recorders. CD-R and DV-R Duplication systems are available in manual towers or in automated systems. You can choose systems that are standalone devices, or come with computer or network interfaces. The automatic duplicators are also available with or without built-in CD printers for printing labels directly on the CD. Most internal CD recorders for personal computers, server systems and workstations are designed to fit in a standard 5.25" drive bay and connect to their host via an ATA, SATA or SCSI bus. External CD recorders usually have USB, FireWire or SCSI interfaces. Some portable versions for laptop use power themselves off batteries or off their interface bus.
CDs were originally designed for audio so it's only natural that CD-R and CD-R/RW recorders write discs in the official Compact Disc-Digital Audio (CD-DA) Red Book format for use in any CD audio compatible player. Just like their mass produced prerecorded (pressed) cousins, CD-R and CD-RW discs can hold up to 80 minutes of CD quality audio (44.1 Khz, 16 bit) using as many as 99 separate tracks. Recordable CDs are WORM (Write Once, Read Multiple) media that work just like standard CDs. The advantage of CD-R over other types of optical media is that you can use the discs with a standard CD player. The disadvantage is that you can't reuse a disc. Depending upon the capabilities of the recorders and software used, CD-R and CD-RW audio discs can be written from either digital or analog sources. Digital material such as existing MP3 files or CDs are conveniently read directly from the hard drive, recorder or from a separate CD or DVD-ROM drive.
There are two basic ways of writing to a CD-R. Disc-at-once (DAO) writes the entire CD in one pass, possibly writing multiple tracks. The entire burn must complete without interruption, and no further information may be added. Track-at-once (TAO) allows the writes to be done in multiple passes. There is a minimum track length of 300 blocks (600K for typical data CDs), and a maximum of 99 tracks per disc, as well as a slight additional overhead associated with stopping and restarting the laser. Recording CD to CD is much simpler than recording from analog sources since most CD and DVD-ROM drives are capable of transferring audio directly (digital audio extraction) without the necessity of converting from analog to digital. As a result, CDs can often be recorded disc to disc using a CD or DVD-ROM drive as the audio source. CD recording on personal computers was originally a batch-oriented task in that it required specialised authoring software to create an "image" of the data to record, and to record it to disc in the one session. This was acceptable for archival purposes, but limited the general convenience of CD-R and CD-RW discs as removable storage medium.
References: Andy McFadden's CD-Recordable FAQ, Understanding CD-R & CD-RW, CD-Recorder and duplication systems, Wikipedia article "Optical disc recorder"