The AC line input is typically protected with low pass filters designed to block radio frequency interference from being conducted back out the power line. If the line input is 240 VAC, then the DC obtained from the filter capacitors is sqrt(2) times this value, or about 340 VDC.
The 340VDC is then fed to high frequency high power switching transistors that "chop" the DC into high frequency AC, typically at tens or hundreds of kilohertz, and fed to a transformer wound to give the desired voltage at the output. The secondary is then rectified and filtered again to produce the desired DC output voltage.
Why use high frequency? Because high frequency transformers can carry far more power for a given weight and size than those operating at 50 or 60 Hz. Also, the filter capacitors at the output of the supply can be much smaller than those operating at 60 Hz.
The power transistors have another function. Although they are always either "on" or "off" at any given instant, the ratios of their "on" and "off" times can easily be varied. Because the filtered output of the supply effectively follows the average "on" percentage (known as the "duty cycle"), the controller can vary the output voltage (and power) over a wide range. It also lets it adapt to varying AC input voltages. And it does so with very high efficiency: switching power transistors dissipate zero power when they're off, and very little when they're on (how much depends on their internal resistance when "on".)
Switching power supplies have been universal in personal computers since the original IBM PC, and they are rapidly displacing the older linear supplies in most other types of electronic equipment.