Definition :

NOT gate is the simple logic that getting true gives false as the output. Just invert the input for example if input is 1 then the NOT will give 0 and if 0 as input 1will be the output of a inverter.

A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.

"If false as input True will be output and if input True output is False"

Truth table :

NOT gate by RTL :

Two resistors and a transistor can construct a NOT gate logic. See the Fig-2, where the base of the transistor is the input terminal after the resistor R1 and the collector side of the transistor as the output terminal.

CMOS NOT gate :

Two MOS transistors are used to construct a CMOS NOT gate. As the Fig-3 a p-type MOS and a n-type MOS connected together Gate to Gate and Source(p-type) to Drain(n-type). Here the both Gates are jointly acts as input and the Source-Drain connected node gives the inverting(NOT)output.

NOT gate analogy :

Fig-4 shows an analogy of NOT logic gate. Which constructed by a battery, a switch(SPST) and a electric bulb where the switch indicates NOT input and the electric bulb as the NOT output. If the switch is closed the bulb will be dark. If again switch A is opened the output the bulb will be bright. That means this circuit follows the Truth table of a NOT logic gate.