Round shiny things made of aluminium and plastic. Data is encoded onto them in the form of tiny indentations called "pits" (for the 1-bits) and gaps between called "lands" (for the 0-bits). The bits are read using an infrared laser.
Note that the data is encoded in a very thin layer just underneath the label side of the disc; any damage to this side can cause irreversible loss of data. When you look at the shiny side, you are actually looking through the layer of transparent plastic which makes up most of the thickness of the disc, towards the data layer under the label. Scratches to this plastic layer can be tolerated to some extent, and there are even ways (though not necessarily reliable ones?) to repair scratches by smoothing them out.
Standard discs hold 650MB or there abouts, although 700MB discs have now become more common.
The physical format of a CD and the representation of audio tracks is defined in a specification called the "Red Book". Discs and players that are certified to this specification carry the familiar "Compact Disc Digital Audio" logo, which is controlled by Philips.
The raw error rate of reading the zeroes and ones on the disc is about 1 in 1000; in audio terms, that would translate to a signal-to-noise ratio of 60dB, which is about comparable with the old audiocassettes (i.e. it would not be considered "hi-fi"). However, by the use of error-correcting codes (specifically, "Reed-Solomon Cross-Interleave Redundancy Checking"), this error rate drops to 1 in 1000000, which translates in audio terms to a signal-to-noise ratio best described as "fantastic". However, in computer terms, that is still not reliable enough for storing computer data.
Many forms of CopyControl degrade the audio quality or alter the data structures on the plastic wafer in an attempt to prevent customers using the data. By thus degrading their product, the manufacturers may no longer meet the "Red Book" standard, meaning they cannot legally be called CompactDiscs. Since consumers might notice that plastic wafers which do not work in their equipment are missing the "CD" logo and so stop buying such CopyControlled wafers, there is an incentive for manufacturers to mislabel their discs.
So be on the lookout for things that are not CompactDiscs which are labelled illegally while shopping.
The "Yellow Book" specification takes the original audio CD, adds some more error-correcting codes to make things reliable enough to store computer files, to produce a disc format called "CD-ROM" ("Compact Disc Read-Only Memory"). The normal filesystem used on a data disc is ISO9660. Hence the common use of "ISO" to refer to disc images and of the extension .iso for their filename.
# mount /path/to/image.iso /place/to/mount -o loop
There are two competing standards to allow longer filenames and a few other things ISO9660 does not provide. The earlier one, originating from the Unix environment, is called Rock Ridge. The other one, made up by MicrosoftCorporation, is called Joliet. (Note that some characters that are valid on Unix FileSystems are not allowed on Joliet discs.)
Under a Linux OperatingSystem, these raw devices are called /dev/hdxn for PATA drives, /dev/scdn for SCSI drives and /dev/srn for SATA drives. Under FreeBSD 5 and later, they're called /dev/acdn for PATA drives and ??? (AddToMe) for SCSI drives.