Why A switchmode (switching) regulator chops the input into a train of pulses at a high frequency and feeds them to an LC low-pass filter. The filter averages the pulse train, so the DC output is roughly the input voltage times the duty cycle. Feedback adjusts the duty cycle up or down to hold the output constant, and because the switch is either fully on or fully off it dissipates little power, giving efficiencies of 85 percent or better.
Watch out Varying the conductivity of a pass element describes a linear regulator, where a transistor acts like a variable resistor and burns off the excess voltage as heat.
Switcher = switch plus filter: output equals input times duty cycle. Linear = pass element throttled like a valve.