Media Transfer Protocol. Two important things about it, one, you can take pictures in a programmable fashion straight from the computer, and two, the camera does not just play dead and act as a mass storage interface That way when you are connected, the camera is still fully operational and can take/delete pictures without tripping over the host connection (not to mention it's able to handle thumbnails and metadata without having to copy over the whole image).

For some reason firewire (even 1394a, not to mention 1394b) is a white crow around here, so it wasn't really an option