lake What is a Calling in The Host?

I just started reading The Host by Stephanie Meyer, and I'm confused beyond repair. I have two questions actually. 1.) What is a Calling? They mention a Calling with a capital "C" and I wonder what it resembles and what it means. 2.) Is the Seeker and the Healer in the beginning (remember im on pg. 27) people or aliens? Or souls? Wanderer mentions hearing footsteps, and describes descriptions of them to be humans. If they are humans then why are they helping these aliens (souls) to take over the human race??? Any help to these questions will be great. Please don't tell me to read on, I just really want to know! : ))there