The Host, by Stephanie Meyer?

I just got this book a few days ago. I am on chapter 9 and I am having trouble understanding what's going on. Can anyone who has read the book explain to me what it means to be a host and a soul? And also, what's up with the planets? Did "wanderer" come from a different planet as an alien,soul or human? And what happened to melanie? I know that she died and that she is from earth, but why does wanderer use her as a host body?