The NDEV (product code RVT-001) is the main development system for the Wii, taking the form of a large black box including full Wii hardware with 128MB of RAM as well as debugger and Host I/O hardware.
The NDEV features all the ports and functionality found on a retail system, in addition to additional communication ports for debugging and host communication. It does not have a disc drive or internal hard drive, so all software is read from NAND or the host system. The NDEV cannot boot without an attached host system, and can only launch disc titles if they are emulated by an ODEM (optical disc emulation) program on the host system.
The difference between 1.x versions is unknown. The difference between 2.0 and "2.01(ES)" is believed to be the addition of wireless functionality, as an official Revolution SDK document describing compatibility for various Wii Remote models states that wired controllers are intended for the 2.0 and wireless controllers are intended for the 2.01.
Between 1.x and 2.0, the differences are:
- Changes in NDEV text size
- Addition of crypto support to IOS
- Likely addition of a boot program (with visible output to the TV)
- (Unlikely, rumor/speculation) Removal of second SD card slot
- Addition of wireless functionality
- 2.01 (ES)
Between 2.0 and 2.1, the number of wired Wii Remote ports went from 4 to 1. This is the main way to identify a 2.0 (rare) from a 2.1 (very common).
Some NDEV's on ebay came boxed, and looked like this with some... interesting info/stuff, I must say.