This Guide is to help with the differant types of Mini Disc that have been out since MiniDiscs were announced in 1991 by Sony as a disc based digital medium for recording and distributing consumer audio that is ``near CD'' in quality, using a compressed audio called ATRAC (a type of Mp3).
The introduction time was just ten years after the introduction of the Compact Disc (CD), CD has effectively replaced the vinyl LP records in the audio disc market.
In terms of function, think of Minidisc as a cross between CD and cassette tape. Like cassette you can record from any audio source and playback through headphones or a home Hi-Fi.
Like CD the audio is stored digitally, the sound quality is excellent, and you can easily skip to your favorite track.
Unlike either CD or cassette you can edit the recorded audio to your heart's content, dividing it into tracks, naming tracks and moving them around, and deleting what you no longer need.
There are two physically distinct types of discs: Premastered MDs, similar to CDs in operation (no longer in manufacture), and Recordable (Blank) MDs, which can be recorded on repeatedly. The disc itself is enclosed in a small (7cm x 7cm), convenient, cartridge.Recording is in real time there for in stereo 80mins of misic only can be recorded on a 80min disc. 
To date in the UK (October 07) Standard 80min blank disc's are still available from the likes of Tesco's and Sainsbury's (Sony 5 pack)
Minidisc "Long Play". Introduced in September 2000 it is a new encoding method for audio on Minidisc that offers two extra modes: one gives 160 minutes stereo ("LP2"), the second gives 320 minutes stereo ("LP4"). LP2 is stereo encoded (each channel is independent), LP4 is joint-stereo (both channels are coded simultaneously).
Discs recorded in LP2 or 4 will not play on older players without this facility. Older disc recorder at standard speed will play on any player. Recording apart for deck with high speed CD - MD transfer is in real time.Until the introduction of the NET MD transfer from a PC can be only be done on some units with a special PC Link Kit (M-Crew softwear) but again this is in real time.  Look for the LPMD mark on the unit.
June 27, 2001 - Sony announced a new application interface called "Net MD" to connect MD recorders to PC via a USB socket. By employing a number of security technologies, the Net MD interface (Sonicstage software) made it possible to transfer music data at high speeds from PC to MD (up to 64x for LP4 audio on certain units) look for the NETMD logo on the unit.
Unless you have NetMD equipped Minidisc gear however, the only route onto or off of a Minidisc is still through real-time recording and playback (except for the high-speed CD/MD decks). And NetMD or not, you can't get at the compressed audio directly from a computer. This is the biggest functional difference between MD and computer connected MP3 players and CD-R burners.
Minidiscs have proven their advantages over lots of other competing products (one of the best things going for Minidiscs is that it is well suited for off air real-time recording from your tuner/computer or any other source with a audio output).
 Although developed by Sony Mini discs have been available from many other Hi-Fi manufactures most have now ceased production due to the Mp3 type technologies developed by the computer market which in the portable field has just about take over from most other formats (to date Oct 07 in the UK Sony still offer a Limited range of MD products)
Introduced by Sony in January 2004, Hi-MD is a nearly complete revamping of the original Minidisc system. The most significant change is the introduction of Hi-MD media, which includes a new 1GB blank with up to 45 hours recording time which can only be used to this capacity on a Hi-MD recorder.
Discs recorded in Hi-MD will not play on older players without this facility but recorders will maintaining playback compatibility with the older types MD discs. Older discs can also be rerecorded (formatted) on a Hi-MD recorder to Hi-MD which will double there capacity

Late Hi-MD will store and play pure Mp3 all Minidisc before the introduction of Hi-MD and the first range of Hi-MD all have to convert the Mp3 to ATRAC and this cannot be transfer back to Mp3 so they cannot really be called a true Mp3 player.