Consider a p-channel and an n-channel MOSFETs connected between power supply and ground. The voltage at their drains is the inverted voltage at their gates. The two transistors behave as a voltage divider. The channel resistances are controlled by the input voltage at the gates. When the input voltage is high, the bottom channel has small resistance, the top channel has large resistance, and the voltage dividers output is low. Similarly, when the input voltage is low, the voltage divider is such that the output is high.
Logic circuits like this CMOS inverter are the most basic building blocks of digital circuits such as microprocessors. Note that most of the current flows when the inverter switches its state. Overclocked CPUs consume more power (and dissipate more heat) because they switch more often.
