This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
MiniDiscs, or MDs, are small (64mm) discs that hold about 140MB of data or
160MB of audio. By using sophisticated compression techniques they are
able to compress audio by a 5:1 ratio, allowing a capacity of 74 minutes
with little or no audible difference in quality. As with CD recorders,
there are MD recorders that connect to your computer and MD recorders that
connect to your stereo.
There are stamped MDs that are similar to CDs in construction, and
rewritable MDs that use magneto-optical technology. Audio MD recorders
are generally more convenient than stand-alone audio CD recorders, because
the playback mechanism allows a more flexible layout of audio data, so it's
possible to delete a track from the middle of the MD and then write a
longer one that is recorded in different places across the disc. The
current generation of MD technology is unlikely to replace CD-R or DAT,
however, because the lossy compression employed is disdained by audio
purists. MD is more often positioned as a replacement for analog cassette
tape, which it matches in portability and recordability, and surpasses in
durability and its ability to perform random accesses.
Computer-based MD recorders can write data, but may not be able to record
audio. Check the specifications carefully.
A wealth of information is available from http://www.minidisc.org/. If you
want to transfer CD to MD or MD to CD-R, check there for more information.
(It used to be item #37 in the FAQ, but doesn't seem to be now.)