The voltage applied at the gate terminal regulates the flow of current between the source and the drain. In this way, a MOS transistor may be viewed as a simple on/off switch.
Assume that '1' or POWER (PWR) or VDD denotes a high voltage normally between 1.5 to 15 volts, and '0' or GROUND (GND) or VSS stands for a low voltage normally set to zero volt. The "strength" of a signal denotes its ability to source or sink current. The strength of '1' and '0' signal can vary.
Figure 1.2 : Switch models of MOS transistors