The DONOFF is controlled by an ESP8266 (ESP-01 board) microprocessor. The main reason to use an ESP-01 (and not e.g. an ESP-12) has everything to do with safety. You can remove the ESP-01 from the DONOFF and program it somewhere far from the dangerous mains voltage (always remove the entire DONOFF from the mains when you disconnect the ESP-01!). Once equipped with the DONOFF firmware, the ESP-01 can also be provided with new firmware 'Over The Air' (OTA) (so without a physical connection)!
The ESP-01 has one input pin (GPIO2) for connecting an external switch and one output pin (GPIO3/RxD) where the PWM signal to the optocoupler comes out. There is also a built-in LED on the board (GPIO1). The ESP-01 works on a 3v3 voltage.