What was the key???

I'd recommend checking out the end of s5, it had some great moments, but anyway here's the wikipedia explanation:

"The Key was created within another dimension with the ability to annihilate the barriers that separate one dimension from another. The longer the Key is in use, the more walls that are torn down. When the Key is activated during its moment, a small portal is opened, which immediately allows anyone to cross from one dimension to another. The longer the Key is used, the more the portals open, the more barriers will come down and eventually dimensions will bleed into each other, forcing Chaos to reign on Earth."

It basically tears down the walls separating alternate dimensions, letting all the demons and dangers from those worlds into ours.

I dont know if I am right about this and I am soooooo sorry if I'm not, but I thought that Dawn was the key of destruction or something like that. Like she was born/created to be sacrificed at the top of that tower, and that was why Buffy jumped of the off it to save her and the world.

Sorry if that is wrong or dosent make sense but that was what I have always thought it was about.

i think she was the key that opens the gateway to hell but im not sure.

It's not really just one hell, but many dimensions. Fused in Dawn's blood is the key to unlocking the walls between dimensions.

Yes, dawn was the key. Her blood had to open Glory's hell portal (otherwise known as "the lock") and while her blood was dripping, the portal opened and hell was released on earth. And the portal wont close unless all dawns blood has been leaked out. (so until she's dead.)

Im not sure if I should tell you the ending. and I know alot people have answered this already, but i like explaining things.

dawn or a ball of light

dawn