An inverter Schmitt trigger is a device that converts any form of an input signal into a digital output signal, so it can function both with digital inputs as well as analog.  The advantage of an inverting Schmitt trigger is that it is also a hysteresis device, meaning that whatever signal is being outputted at the current moment has an effect on any future output.  This means that there is not a proportional or constant output for any one input state.  In a Schmitt trigger, there is a wide range of input signals that can result in one of two different output states.

For an inverting Schmitt trigger, there is a high and low output state possible at any one time.  When the input signal produces a voltage that crosses a certain high voltage threshold, the circuit goes to its low voltage output state and remains at that state until the input signal reaches a certain low voltage threshold, at which point  the output will switch to a high voltage and remain there until another threshold is reached.  This means that the Schmitt trigger’s output is completely immune to any input changes as long as the voltage of the input signal lies between the upper and lower voltage thresholds.

Schmitt triggers are a form of bistable multivibrators.  This means that they have two stable states that remain fixed until an input signal changes them.  This has many important applications, especially when creating certain oscillator circuits.