The power supply typically provides 12--24V to the stepper driver chips (usually mounted on a gShield) --- some power supply setups will provide a separate 5V which may be useful to power the microcontroller for headless operation.
(The following text is repeated from the Electronics page)
The selected power supply should produce 4.2A - 6A at 18-30v for NEMA17 motors, NEMA23s want 8A.
At a very minimum, you need a 90 W power supply for four NEMA17 motors driven from a gShield with three stepper motor drivers (with the two Y motors in parallel on one driver). For example, a 24 V 3.75 A supply will work, but the ideal supply would have a little more headroom -- say, 24 V 4.2 A (100 W). You'll need 100-120 W to make the most of a four-driver board, such as the buildlog.net shield or one of its clones -- for example, a 24 V 5 A supply. If you have NEMA23 motors, how much power you need and how much your gShield can get out of them depends on their impedance and rated current. You need 150-200 W for four small (50 mm) NEMA23 motors rated 1.5 to 2 A. With lower impedance motors (such as the 2.8 A motors sold by Inventables as of July 2014), the gShield won't be able to achieve the rated current, so a smaller supply will do (say, 120-150 W). It may be a good idea to connect the Y motors in series in this case, or to use a four-driver shield.
110 Volt draw
That power supply is 10 A output, at 24 V, for a total of 240 W. Ignoring losses and power factors, that translates to 2 A at 120 V (or just over 1 A at 230 V) at the input. Taking losses and a typical (bad) power factor into account, 4 A would still be plenty.
Voltage and Wattage Guidelines
NEMA 17, 100-120 W should be sufficient; for the typical 50 mm long NEMA 23 motors, you probably need 150 W (or 200 W, which may be easier to find).
Replacements are listed on the Parts page.
Notes on wiring up a power supply:
Some power supplies will include a potentiometer which may be adjusted to control the amount of power which it provides.
XBox 360 power supply --- this requires a special adapter