Inside your PSU
 

The components in a switchmode PSU that let it do what it does are Metal-Oxide Semiconductor Field-Effect Transistors, or MOSFETs. Transistors are solid state switches that can operate much faster than any mechanical switch; that's why CPUs are made out of transistors, not relays. MOSFETs are high power transistors; they can switch a lot of current. This makes them useful for power conversion.......

Switching regulators don't actually change the voltage going through them. They just turn it on and off very quickly. Feed a switching regulator 20 volts and ask for 10 volts from it, and it'll rapidly pulse its output so that half the time its output is at 20 volts and half the time it's at zero. Do this fast enough, and put a capacitor or three across the output to smooth it out, and you get a nice clean ten volts without ever having to just throw watts away as heat..........

http://dansdata.com/psus.htm

That's what normally called "gates". ;)
Another nice article to learn about electronics from Dan.

Think of a door that must be open and shut so rapidly and the energy requires to do the job ..... the energy to do the job is waste which becomes heat. Hence, MOSFETS heats up; but cheap to produce in mass quantity.