Originally Posted by http://www.bleedinedge.com/guides/psu_select/psu_select_01.html
A rail is the delivery system for power. On a modern PSU these rails are divided into +3.3v, +5v, +12v, -5v and -12v. The important ones for us to look at are the positive rails. These will be specified to deliver an amount of amps per rail. Seeing as w = v * a. This tells us the output in watts of the PSU.
* Watts =+3.3v rail*30a = 100w
* Watts =+5v rail *30a = 150w
* Watts =+12v rail*25a = 300w
Adding them together gives us a power output of 550w. There are caveats to this, which we will examine later.
The modern trend of components drawing off the +12v rail, has led to the introduction of multiple +12 rails, which are easier and cheaper to produce that bigger rails, but more importantly easier to keep stable and cool.