When you want to know how someone is in bed, but can't sleep with them for any number of reasons, and you talk a friend into doing it for you and reporting back.
Lola: Damn I wanna have sex with Aron, but Brad would KILL me!!
Holly: Yeah, he is really hot! I would totally do him!
Lola: Awesome! Will you be my changeling and report back?
People who will frequently show up in an online community, burn every one of their bridges, start arguments, troll like it's the end of days, garner universal disdain from their fellow board-members, then get booted from the board, only to show up again immediately afterward under a new user-name, and continue to repeat the same process, ad infinitum.
BloodNinja: Hey babe, wana cyber?
BritneySpears14: No! Get away you freak!
eminemBNJA: Hey babe, whats going on?
BritneySpears14: Ugh, another changeling.
A girl who is sometimes attractive with make up on but completely busted without!
Mike: Hey did you see Maureen last night without make up!?
Chris: Ugh! CHANGELING!!!