NOR stands for Not OR, and comprises a Not (inverter) and an OR gate in series. It is a digital integrated circuit, not to be used in analogue situations.

The states of the inputs that lead to certain outputs are displayed in the truth table:

In A In B Out
0 0 1
0 1 0
1 0 0
1 1 0

Only when all inputs are low, the output turns high.