NetMD is an added feature in minidisc players that enables them to record tracks to the disc directly from a PC using it's USB port, in faster than real-time speeds. (not to be confused with a Minidisc PC-link, which enables a digital input straight from the PC's USB port, but only in realtime speed.)
The NetMD host application converts digital audio formats such as wav and mp3 to the minidisc ATRAC or ATRAC3 format. The download times alternate between different ATRAC compression rates (SP - 4 times faster than realtime, LP - 16 times faster, LP4 - 32 times faster).
NetMD recorded minidiscs will still play in players that do not have the NetMD feature.
NetMD is very nice, but not perfect - the download speeds are much slower than mp3 players, not to mention the lightning fast iPod, it doesn't work with Apple computers, and you can't upload tracks from the minidisc unit to the computer.