what people mean when they say CMOS is an area of memory on your motherboard that holds some system config info, like the date/time, what drives you have, memory speed settings, bus speed settings, and various others, even when the computer is unplugged (there is a battery on the motherboard). if the computer doesn't boot right, sometimes clearing this by shorting the battery will help.
POST = power on self test. technically, it is everything you see until the windows logo comes up, while normally people just mean up to/including the memory test.