MiniDiscs are a disc based digital medium for recording and reproducing primarily audio but also other data. The audio variety were first marketed by Sony in 1991, with the computer data variety (MD Data) arriving in 1993.
They have gained rapidly in popularity in recent years due to their excellent audio quality and convenient, durable and light-weight design. The disc itself is enclosed in a small (7cm x 7cm), convenient, cartridge.
The audio quality is often described as "near CD" (a fairly useless phrase), but in my experience they have a depth and punch that seems sadly lacking in CD's and is nearer to the feel of a good quality magnetic tape recording. These things are very subjective and so you should make your own mind up. Unless you are especially fussy or inclined to audiophilia you are unlikely to have any complaints especially with the 24 bit models.
There is now a large community of MiniDisc users on the Internet and so rather than attempt to cover the subject in full on these pages we will point you to some of the other sites offering a wealth of information.
MiniDisc Community Portal