What are the available kinds of power supply?
There are two main kinds of power supply available, we will focus on the Switch-mode because of its efficiency, but is always good to know the other option.
Switch-mode power supplies:
Nowadays practically all power supplies within the power range required by RepRap printers (250~350W) are switch-mode power supplies (as opposed to linear power supplies, which are considered technologically outdated).
Switch-mode power supplies have relatively complex circuits that convert mains AC electricity to the DC voltages required by RepRap steppers and electronic circuits. The main advantage of an Swith-mode power supply is its high-efficiency in converting energy. for more information you might want to take a look at this full explanation
There is also this video, it talks about step-up converters, they are NOT the same, but the logic behind it is, and its easier to understand:
Linear power supplies:
This kind of power supply usually use a transformer to reduce the AC voltage, to about 16V AC, then a rectifier circuit will take care of converting the AC voltage into DC, and a stabilizer circuit will make it stable.
The disadvantage of this system is its low efficiency, high cost and taking huge amounts of space. You can read more about this here