A CD-R (also called Compact Disc Recordable) is a compact disc that can be recorded once. When people record a compact disc, they put either music or data on it, or sometimes both, which is called an enhanced CD.
A compact disc that can be recorded multiple times (and erased) is the CD-RW.
When you put data onto a CD, it is called burning a disc. A laser "burns" pits into a dye layer on the disc, making them transparent. These transparent pits can later be read back by the CD drive or audio CD player as data or music.
The first CD-Rs from the early 1990s held 63 minutes of audio. In the mid-1990s, 74-minute discs became common. Most CD-Rs made from 2000 onward hold 80 minutes, or 700 MB of data. 90 and 99-minute CD-Rs are available, although they are non-standard and many players and recorders do not support them.
There are three types of dye used in CD-R discs. The most common is phthalocyanine, and it is usually light green. CMC Pro (formerly Taiyo Yuden) uses cyanine dye. It is usually teal or dark green. Verbatim uses phthalocyanine dye on some discs, and AZO dye on others. AZO dye is usually dark blue or blue-ish silver. The metal layer on the disc is usually made of silver. Archival discs and some professional audio discs use a gold top layer. Verbatim UltraLife discs have a silver main layer, and a gold upper protective layer, providing the reflectivity of silver and the chemical stability of gold. Early cyanine discs could decay and become unplayable within a few years. Recent cyanine discs have preservatives added to the dye to prevent this from happening. AZO and phthalocyanine dyes do not need preservatives and do not decay easily. Phthalocyanine is the most stable and longest-lasting dye, but it requires a more precisely calibrated laser than AZO or cyanine to get a good, error-free burn. A recording laser with too much or too little power will produce a poor recording on phthalocyanine dye.