Sony introduced Minidisc as cheaper alternative to
CDs. First Minidiscs were made to store audio in digital format, one
disc contained 74 minutes of audio. Later data Minidiscs have been
Minidiscs were cheaper than CDs because CDs had to be manufactured using very strict regime to comply with durability and longevity standards. Unfortunately many companies made CDs using less strict regimes and that's why you can't read CDs you've recorded 5 years ago, but 15-year-old CD reads well.
There is quite significant difference between recordable and non-recordable Minidisc. Non-recordable Minidisc is very similar to small CD enclosed in caddy with sliding door. Recordable Minidiscs are magnetooptical media, which means that it's needed to supply heat (by laser) and magnetic field to record data. To read, only laser is used (Kerr Effect- see FAQ). This method allowed Minidisc to be written many times, so it was not like a CD-R, write-once medium.
There were players, recorders and data drives for Minidisc cartridges.