Yep it is exactly as ignatzatsonic says mate.
The mains coming in is stepped down from whatever your mains voltage is say 240v AC like us in the primary
winding (a coil around an iron core) . This induces
a a lower voltage (usually and for our purposes) in a secondary
winding (also around that iron core) - but they do not actually meet.
The 12v 5v and 3.3v (AC) supplies originate from that secondary winding on the transformer at differing points in that winding (tappings).
Now the current from those secondary windings goes through a circuit which in very simple terms rectifies the AC (alternating current) to DC ( direct current) and also boosts the current output to a much higher current than it is at the transformer tapping. (The tappings are AC and usually a slightly higher voltage than the DC output because of some energy loss in the rectification and transport process) Depending on the quality of the transformer and the other circuitry
you get a DC voltage / current for your machines components. The better the quality and design of that circuitry the more you will get as usable wattage.
You can calculate the wattage by simply multiplying the current by the voltage (stated on the PSU specification sticker) and you are always or I am guided by the specs of the 12v rail current. ie 12v X 25 amps = 300watts.
For pure curiosity's sake it is exactly the same formula for the other rails.