well in most MMO's (or at least the one i am familiar with)
tank items usually have physical resist and mage items have magic resist
where as in league:
armor does nothing.
and every champion can stack magic resist
mages/assassins are supposed to counter all squishy targets
Proper league meta"
adcs are supposed to counter tanks (although frankly they last like 3x longer)
bruisers/juggernauts are supposed to counter anything they can get in range on
tanks/ support are supposed to provide utility and healing / disruption
and aren't really supposed to "win" any fights against another class unless there is a significant gold disadvantage on the other end
however right now its
ADC counters everything
tanks/bruiser junggernaut counter everything but ADC
assasins counter mages and supports
mages counter support and anyone with less than 2k hp (lvl 18 full build) that is isnt an ADC