Technical summary
Basically, a 25V voltage sensor, like the one used here, has 5 pins in total. Two of them are on the two-pin terminal and three are male pins.
The screw terminal pins are marked VCC and GND and must be connected to the external voltage source, i.e. the voltage to be measured.
Coming to the three male headers, they are marked as S, + and –. The S pin is the "signal" pin and it must be connected to the analog input of the Arduino. The "–" pin must be connected to the GND of the Arduino. The pin marked as "+" is not connected to anything (it is an N/C pin).
