I'm very interested at the ATmega328, Arduino BT is a microcontroller board . However, I cannot find the BT library in Arduino website. Could you give me a link of the library and some example code?
The Arduino BT doesn't need a library specific to Bluetooth. On the board, rather than connecting digital pins 0 & 1 to USB as in other Arduinos, these are used for transmit and receive on the inbuilt bluetooth module. This is serial data, sent and received just like the Arduino models that have a wired interface using the Hardware Serial functions - of which you'll mostly need to know about begin(), print(), println() and read() to get started.
Have a read of the ArduinoBT Guide page for more detail on how to communicate with the PC and program an ArduinoBT.
Cheers ! Geoff