I read some articles about the AT commands but I am still unable to understand something. As the article says "AT commands are instructions used to control a modem." I used a while ago an Bluetooth module(HC-05) to control a RC car and I saw that the module recognise AT commands. I could control the car using only UART communication and at that moment I just ignored the AT commands. So, why does the Bluetooth uses AT commands ?
As far as I know you can configure the HC-05 module with AT commands. Sometimes you want to change the baud rate of the module, the role (master - AT+ROLE=1 /slave - AT+ROLE=1), to get module Bluetooth address(AT+ADDR?),or to reset the configuration (AT+ORGL). Take a look here to see how to enter in AT mode and how AT commands look like: http://www.linotux.ch/arduino/HC-0305_serial_module_AT_commamd_set_201104_revised.pdf