A proprietary open wireless technology standard for exchanging data over short distances. Used by the NXT for communication between the programmable brick and a PC or Mobile device.
The nxt brick can use Bluetooth to communicate with computers and some mobile devices*.
For support and advice with using this, see the official LEGO Support Page:
* The LEGO Group have released an official app for Android phones. The LEGO Group would need to get the NXT Brick certified by Apple if they wanted to connect it to iPhones.
As the Android app is open source, it's possible that someone might port it to Windows Phone 7 at some point.