The Schmitt trigger is a circuit that only triggers when the input changes sufficiently. The reason why there is a special circuit for this is that there are instances where a small change in input causes a circuit to interpret this as a change, when it should ignore it.
There are two instances where this could be a problem. One is from a slowly rising and/or falling wave. The second is from a noisy signal.
An example of such a scenario is a digital circuit that has an input that is noisy. There may be a circuit that it is connected to that needs to respond to binary 0 or 1. If the signal is noisy then this circuit may interpret it as 1, when it is in fact a 0 with a number of fluctuations. The Schmitt trigger would resolve this by only responding to the significant changes and ignore those that are deemed to be insignificant. In this manner, a Schmitt trigger can be thought of as a form of signal conditioning.
A Schmitt trigger can be built from discrete components but they are also found in ICs. In the 74 and 40 series there are a number of ICs that have Schmitt inputs, including: inverters, buffers, NAND gates, monostable multivibrators and bus transceivers
A Schmitt trigger works by providing hysteresis. If the input level goes above the lower threshold but is below the upper threshold, then it stays where it is.
The symbol is derived from what is known as its transfer function. A transfer function displays the input and output voltages.
Otto H Smitt