ron baker :
Whats this ps2 power supply nonsense ? I ve heard of atx sfx and sfx-l never ps2 . Thats a playstation.
ATX is a motherboard form factor, not a power supply form factor. There are ATX specification for power supplies, but those concern the way they're wired and have nothing to do with the size and shape.
PS/2 is the a factor developed by IBM before ATX existed. It persists to this day and is generally used for "full-sized" power supplies. One reason this form factor isn't "ATX" is because it's also AT (pre ATX).
PS/2 form factor includes a depth (front to back). Most high-capacity power supplies ignore that part of the specification.
PS/3 is a shorter version of PS/2
SFX is the next common size down. It's because of people like you that Newegg lists most of these as "Micro ATX", even though Micro ATX cases often use PS2 or TFX.
So PS2, PS3, SFX, TFX are the most prevelent power supply form factors.