I know that 2 25A rail = 1 50A but what computer parts use the 12V rail and will I have to get an adapter or something?
Not necessarily. A psu will have a maximum output that is often less than the sum of the rails. One needs to check the data plate specs.
The +12v rails is used for primarily the cpu, graphics cards and peripherals. That is almost all of the power output.
Some cheap psu's advertise high wattage, sending much to the 3.3 and 5v outputs.
Really single/multiple rails is a non issue. In theory, multiple rails limit the current to a single section of the psu for safety reasons.
In actuality, a quality psu has no safety issues regardless of how many rails it was designed with.
And, no, you need no adapters.
Just check how many power leads your components require. If the psu has sufficient, odds are, that it also has sufficient power to do the job.
There are very few "true" multi-rail PSU's in the consumer range. Most of the ones you see are a single rail that has been split to reduce stress at the cost of overall power. If you do go for the multi-rail PSU, make sure that you split the loads evenly if possible, like in the case of a GPU or GPUs that use more than one PCI-e power cable.
pci-e connectors are fed by the 12v rail(s). the 3.3 and 5v rails feed things like HDs, DVD drives and some other low power devices. This is why you want your 12v rail to account for at least 85% of the PSU overall power.
so the rails are not a physical cable? Just the slot where you plug the cable in?
The rails are internal circuits. Most of the time, a psu will actually only have one power generating circuit, but the output is divided into multiple circuits, each with a limited output. Power leads will be attached in an even manner across these circuits or rails.
As others have said multiple versus single is a nonissue. A rail is simply a bundle of wires that are all tied back to the same point on the protection chip and all carry the same voltage. Some "multirail" units actually have the 12 V rails all joined together with no over current protection on them, they list it as multirail because Intel used to require multirail units, its now just a suggestion but many people dont trust single rail units so they make it a multirail unit on the label but single rail inside to keep costs down.
Also, as others have said, one 25 A rail and another 25 A rail will rarely add up to 50 A available between them.