Automatic pet doors generally utilize RFID (Radio Frequency Infrared Device), infrared, magnetic or electro-magnetic technology.
All automatic pet door models have sensors that pick up a signal from a small transmitter on your dog or cat's collar. Only when the signal is received will an automatic pet door open.
A collar activated pet door is certainly convenient and will also eliminate the possibility of other animals getting into your home whereas a manual flap door is basically an open invitation.