The NOR Gate has 3 Inputs and 1 Output. It will output power if it doesn't receive any power from any Input side. It won't output power if it receives power from at least one of its Input sides. The image on the right shows the 3 Inputs that are linked together. Similar gates are the XOR Gate, the OR Gate and the XNOR Gate.
A NOR Gate is used for circuits where multiple power sources can unpower the same thing, such as 3 buttons placed around a house, that cause a piston to open the entrance when pressed. When using a NOR Gate, using an Invertor is no longer necessary in combination with an OR Gate. It will give the same outcome, however, the NOR Gate gives the advantage that it needs little space, less material and is less complicated.
0's represent an inactive signal. 1's represent an active signal.
Note that the outcome is equal to the Output of the NOR Gate.