In a PC, different components use different voltage levels, and so a power supply must take the electricity from the wall and split it into 12v, 5v, and 3.3v power. A rail is simple a wire/path inside the PSU that carries electricity of a certain voltage. The 12v rail provides power for GPUs and CPUs, generally the two most power-hungry components. It should be noted that a 12v rail can be split into multiple 12v rails, in which case two 12v rails would provide half the wattage of the combined 12v capacity.
"Rails" is the term used to describe individual voltages within a power supply. In ATX12V PSUs 3.3V, 5V, 5Vsb, -12V, and +12V rails are present. Some manufacturers break the +12V into further separations for safety purposes, as described in the ATX12V 2.2 standard. However, many manufacturers discovered the cost of breaking the rail into still-smaller rails that were current-limited was cost prohibitive, and that many new devices (video cards especially) were demanding more current than one of the smaller rails could deliver. As a result, in the ATX12V 2.3 standard, the requirement to separate the +12V into smaller rails for safety was removed. Now most PSUs use a single 12V rail that supplies current to whatever device needs it.
When running multiple video cards in an SLI or CrossFireX array, however, it is reasonable to invest in a PSU with multiple 'true' +12V rails to help increase the longevity of the PSU (and for safety purposes) by more evenly splitting the power load. Due to heat generation from being under a constant and severe load with multi-VGA configurations, having multiple +12V rails will ultimately make for a positive impact on enthusiast rigs or more hardcore setups.
+12V rails deliver power to a few different devices, the most notable of which being modern video cards and the 4/8pin connector for the CPU.
At the end of the day, only the most extreme users should care about the difference between 1x+12V rail and 2x+12V rails.
We moderate comments on a ~24~48 hour cycle. There will be some delay after submitting a comment.