An Acronym for Media Transfer Protocol
A newer protocol used by some MP3 players, such as the Samsung U3 series. (Incidentally, the U3 also plays OggVorbis files.) Such players don't use USB mass-storage, so you can't (yet) see them in your file browser window and drag-n-drop files to/from the device like a USB disk.
On linux, support relies on applications using the libmtp library. Applications that support this include RhythmBox (in GNOME) and Amarok (in KDE). Your distribution might also have a package called mtp-tools for accessing these devices from the command line, if you are that way inclined.