Computer power supplies are rated in their DC capacity, ideally a 600W computer power supply can supply 600W of DC power but only supplies as much as the system needs so its not supplying 600W all the time, it will pull significantly more than this from the wall since its not 100% efficient, normal power supplies are 80-90% efficient these days so to deliver 600W at full load would pull 667-750W from the wall.
Laptop chargers are rated similarly, they supply power at a fixed voltage and variable current, their ratings are the voltage they supply times their max rated current, they too pull more than their DC capacity from the wall when at full load.
For power supplies at least, if the PC is using all the 600 watts, the an 80plus rated power supply will be sucking 600*.20= 120+600= 720w, up to 720w from the wall. Look at calculator for the components estimate watts. http://extreme.outervision.com/psucalculatorlite.jsp , the Watt term or unit is defined as a unit of energy"joule" per second, so it's more like a rate rather than a flat number.
^ +1 for hunter315, to add, most cheap psu's are not rated correctly. they may say 600w, but are unable to deliver that under continuous load and will blow up. Somehow they can get away with blatent lying with their specs, some psu's even have fake 80+ efficiency stickers on their boxes. http://www.hardwaresecrets.com/article/Coolmax-CUL-750B... < theres a review of a 750w psu that blows up if you draw more than 450w from it. If your unfamiliar with PSU's some good reading is from "jonnygurru" and "hardwaresecrets" websites. http://www.hardwaresecrets.com/article/410