power rail is basically how much power the power supply gives to different components in the computer.
for example, rail 12 is connected to the graphic card. some power supplies have rail 12+ and 12- while some only has 12. usually if you're buying a powerful graphics card like a 7900 series, you would want at least 25amps on the 12 rail so it be enough power for the graphics card to operate.
on the power supply there should be a sticker that tells you which rail has how many amps.
e.g. 25A means 25 amps, etc.
hope that helped and welcome to TF.
Intel Core 2 Duo E6420 Conroe 2.13ghz
Gigabyte P35-S3 Motherboard
A-DATA 2x1GB DDR2 800mhz memory
Geforce 8600GT 256MB GDDR3 PCI-Express