Intel did away with the AT standard and it's two separate "P8" and "P9" connectors by making the main connection from the power supply to the motherboard a single piece. Enter ATX.
PSU is "LIVE", but not "On".
ATX power supply is always "live", even when it's "off", when it's
plugged in and it's rear switch (if applicable) is switched on..
+5V standby is always coursing through your motherboard whether
your PC is on or not. This
is how the CMOS keeps it's settings with or without it's CMOS battery
and wake on LAN, wake
on ring and keyboard power on is accomplished.
power supply is instructed to energize the other power leads throughout
the system when the "Power On" lead (the green wire on the main
connector) is grounded. Without
grounding the green wire on the power supply, you will not be able
to test ANY of the power leads on the power supply except
for the +5V standby.
test the power supply, you can do this: Unplug the power connector
from the motherboard. Connect the green and black wires on the
connector with a paper clip or a piece of wire. This will energize
the power supply. Now, using a multimeter, the 12V should
read 12V, the 5V should read 5V and the 3.3V should read 3.3V
(within +/- 5% per Intel specifications.)