The Arduino BT is an Arduino board with built-in bluetooth module, allowing for wireless communication.
In addition, it has some other differences from an Arduino NG:
The bluetooth module used is the Bluegiga WT11, iWrap version (details and datasheet [pdf]). The bluetooth module can be configured with commands set over the serial port from the ATmega168 (see the iWrap user's guide for details). A program to set the name and pass code of the bluetooth module is run once on each Arduino BT. The name is set to ARDUINOBT and the pass code to 12345. For complete details, see the initializiation program's source code. The ATmega168 comes preloaded with a bootloader that allows you to upload sketches to the board via bluetooth. The bootloader source code is available in the Arduino SVN repository.
Reference Design: arduino-bt-reference-design.zip
Schematic: arduino_bt06.pdf