Three CMOS Inverters with feedback
The circuit is essentially this:







simulate this circuit &ndash; Schematic created using CircuitLab

Obviously, the outputs of NOT1 and NOT3 will conflict with each other at times. The point is that the transistors of NOT1 are large enough to "overpower" the transistors in NOT3 when necessary.

Basically, the output of NOT3 functions as positive feedback for NOT2, tending to keep it in the state that it's already in. This creates hysteresis in the transfer function of the overall circuit, also known as a "Schmitt trigger".