The Media Transfer Protocol (commonly referred to as MTP) is a devised set of custom extensions to support the transfer of music files on USB digital audio players and movie files on USB portable media players.
MTP Media Players
A number of MP3 players, such as those produced by Samsung, Creative and many others use Media Transfer Protocol (MTP). These devices, when used with the correct driver, often appear in Windows as a media device but can be accessed as a USB device.
Ubuntu 11.04 supports these devices for instance in Banshee but requires two steps:
Install the mtpfs and mtp-tools packages.
sudo apt-get install mtpfs mtp-tools
Open Applications → Sound & Video → Banshee Media Player.
Click Edit → Preferences.
Tick the MTP Media Player Support extension.
Your device will now be displayed in the left hand pane under Devices when connected.
There are really useful guides on mtp and ubuntu here for some common mtp devices: Ubuntu forums how to guide
Originally MTP was designed to be part of Microsoft's 'Media family' its adoption in devices like those from Creative, Blackberry and Samsung mean that many Ubuntu users struggle to get these devices to sync automatically.
However more recently there are methods to address this using the libmtp shared library that implements the MTP protocol and also includes some command-line example tools.