Volts X amps = watts.
These are often labeled as their peak ratings. I'll just use an example of this delta dps-700 I have here. It has 3 12v rails rated at 18A, and one at 16A. Combined, that should be 70A, or 840W, right? No, as the psu is only a 700W psu. It is just the max that each rail can put out. The total combined amperage of the 12v is 50A.
The wattage is calculated by adding the max combined wattage of the 12v with the max combined wattage of the 5v and 3.3v, negative voltages and 5vsb are typically not included just because the 5vsb is on a completely different circuit and the negative voltages are barely used.
Also VERY important to note that a power supply's rating are unregulated. Companies take advantage of this all the time by stuffing a 250-300W unit in a 650W case and sell it for $20.
Delta is my 1st choice in power supplies
Biostar Tpower N750 (all UCC solid caps) 5000+ dual core PNY 9800gt 4gb OZC DDR2-800 320gb WDC 700W Delta DPS-700