The HC-06 is a slave module, which can only be used to pair with another HC-06 or HC-05 module. The HC-05 is a master module, which can be used to either pair with other HC-05 modules, or to act as a standalone controller. The HC-06 has an external antenna, while the HC-05 does not. This means that the HC-06 can communicate at longer distances than the HC-05. The HC-06 uses more power than the HC-05, so if you're using battery power, the HC-06 will last for less time than the HC-05.