So I think I know the general idea of stacks: each stack is a certain amount of damage, and you get to have a lot of stacks. My confusion is.. what EXACTLY are stacks? How DO you stack? I think it's different for every champion, but can somebody explain this to me? thanks
It depends.
Some champions have abilities that "stack," or gain power by overlapping one another or having buffs that persist long enough to overlap. Champions like Sion, Nasus, and Veigar all have abilities that "stack." Every time they kill a minion or champion in a certain way (with the exception of Sion) they become more powerful. Sion gets points added to his maximum health, Nasus makes his Siphoning Strike more powerful, and Veigar gains ability power.
Other things like Twitch's deadly venom can be applied to the same target multiple times to increase its effectiveness. Vel'koz has his passives that activates when it stacks 3 times, dealing true damage based on Vel'koz's level, Braum's passive stacks up and stuns you when you get 4 stacks....
It's not all that common of an ability, but this is the basics.
It depends on what you're stacking. Bloodthirster stacks with every auto-attack. Tear stacks on spell casts. Soulstealer stacks on champion kills or assists. Nasus and Veigar Q's stack with killing a unit with that spell.
Anything specific you want to know about in terms of how it stacks?
