HC-05 4 Pin Wireless Serial Bluetooth Module is a Bluetooth module for use with any microcontroller. It uses the UART protocol to make it easy to send and receive data wirelessly.
The HC-04 module can be used as a Master as well as a slave device for transmitter and receiver. This means that it can connect to most phones and computers with Bluetooth aw well as to another slave device such as keyboards and other HC-06 modules. To connect with other slave devices a master module would be necessary.