Raid mob resists - clarification needed.

[Netherzul]

I need clarification on this if anyone would care to shed some light.

In classic, I played a Monk and Shaman up to and into Discord briefly. I spent plenty of time soloing mobs on my Shaman but in raids, all I really did was slow/buff/heal. I never spent time dotting/nuking, in any capacity, so I am completely out of the loop on how boss resists work.

Typically, what schools are resisted most, by bosses? I know this question could follow with a response of, "that depends on the boss" but I am asking in a general sense here.

I know some classes (Shaman, Magician, Necro, Ench) get resistance debuffs. Do these stack with one another? If not, which one will take hold over any of the others?

Thanks in advance.