Bluetooth Shield a serial port Bluetooth module (Master/Slave) breakout board. You can choose ARDUINO's Digital PIN D0->D7 as Software UART to communicate with Bluetooth Shield (and D0,D1 also is Hardware UART).
It could be set to Master or Slave by user.
Note: Only between master device and slave device can be paired with each other. Between Master and Master devices or Slave and Slave devices can't be paired each other. The Slave mode could be paired with cell phone, PC etc.