Cgrado has nicely given you an option, thought id just take a moment to just say that the descriptions you see there are describing the types of PSU units, like their architecture. Hmm, here is a wee bit about it - with excerpt pasted here..
there was a standard of ATX 20 pin, then it changed to EPS 24 pin. Modern psu units generally you are looking at ATX 12V
EPS stands for Entry-Level Power Supply Specification, although it seems like one S was dropped from the acronym. EPS was developed by the SSI Forum which is a group that works on server standards and as such EPS is a standard for entry level servers. Intel has not gone for full EPS spec as the four-pin AUX connector is still present, whereas EPS utilises an eight-pin AUX connector.
The advantage of this is that you can get a converter for current ATX PSUs that turns the 20-pin ATX connector in to a 24-pin EPS connector. It is however not advisable to get one of these for anything less than a 350W unit due to the extra drain on the PSU that the new motherboards will have.
The main reason for the changes, according to the new specifications which you can find at Formfactors.org, is the higher power drain from the x16 PCI Express lane, which can draw as much as 75W of power.
Another spec change is the requirement for dual 12V power rails. This might sound odd, but with more and more components in a PC drawing their power from the 12V rail, this change spreads the load across two 12V rails rather than putting a single 12V rail under too much load. This allows for a cooler, more efficient running PSU as the load on the components are shared.
ATX and BTX are different layout forms, like the physical layout of where components in yer case are installed. The majority at this time are ATX, even though BTX was brought out in 2003 i think.