The NOR gate is obtained by combining an OR gate and NOT gate. The output of OR gate, Y′ is inverted by the NOT gate to get the final output Y.

A |
B |
Y' |
Y |

0 | 0 | 0 | 1 |

0 | 1 | 1 | 0 |

1 | 0 | 1 | 0 |

1 | 1 | 1 | 0 |

The Truth Table of a NOR gate can be arrived at by inverting the output of an OR gate.

The Truth Table of a NOR gate shows that it gives output 1 only when both the inputs are 0.