A Development Xbox has a USB port on the back for a keyboard or mouse; it also has a Serial Port for Kernel debugging. Also on the Dev unit, there is a port similar to a Scusi port. It has a extra circuit board on the inside, slightly different cable (internally) 128 mg ram, and the software on the hard drive. A debug is basically a normal xbox, with a pin header already installed on the LPC bus, it has 128 mg ram as well, as a blanked EEPROM. Plus the Debug has a see through green case that says debug on the front, and the Den unit has a clear case with development kit on the front. Also on the Dev unit, the lid sits higher than on the on retail, or debug xboxs to accommodate the "extra stuff". Other than that the software that runs them if basically the same, just you can use more parts of it on the Development unit, such as the Kernel debugging, emulating, and the mouse and keyboard. The Development units are more for software developing, and the Debugs are more for testing, and other things. I hope this helps answer your question.