What was Buffy the Vampire Slayer's sister Dawn before...you know.

Spoilers below, fellow Buffy newbies:

I don’t get it: was Dawn around before the monks created the key? Do Buffy and her mom remember Dawn? Does Dawn have a memory of her past? What IS Dawn?

I’m currently watching episode 12 of season 5, so we’ve been talking about Dawn as the key for a while, but I still don’t quite understand.