the processors are interchangable they use the same socket , but like TMraven said thats the end of similarities to a motherboard , while motherboards are standardised atx or mini atx or micro atx even in most all in one pc's
apple is using their imagination which shape of logicboard will fit best behind the screen
so while on a all in one pc with atx board you can after a fail just use any motherboard you like ,down-or upgrade to any board you like , in iMac's you have no such option you need the exact apple part , thats why it is important to take out apple care , so you are safe for 3 years
random tip : if you buy a iMac buy it with apple care , if it is has not developed faults after 2 years 11 month sell it quickly and buy a new one
(now start at beginning of tip again )