Switching power supply
They are generally more efficient, but are also more complex, than the linear power supplies which preceded them. They are thus now found ubiquitously in medium- to high-power applications.
In general, all switching supplies depend on turning the input current off and on quickly; voltage regulation is achieved by leaving the input on longer when the output voltage is too low, and leaving it on for a shorter time when it is too high. Passive circuit elements such as capacitors are used to filter out the ripple and noise from the switching.
The transistors usually used as switches are thus operating in either fully-on or fully-off mode; they are more efficient when used in that way, contributing to the efficiency of switching supplies.
There are a wide range of design approaches in the switching supply field, depending on factors like whether isolation is needed (i.e. from wall AC, which generally requires a transformer); the need for VI conversion; how good the voltage regulation needs to be; cost and size goals; etc, etc.
Isolation, VI conversion, and regulation are different objectives, but they can be achieved either separately or in concert; e.g. use of a transformer can do the first two in one step. A buck converter is often used for DC VI conversion, when going from a high voltage to a considerably lower one.
One common design is to take input AC, convert it to higher frequency, run that through a transformer (since transformers can be physically smaller and lighter at a higher frequency, for a given power throughput), and then convert the output to DC.