It was explained to me once like this. "Heaven" and "Hell" are not places, but states of being. For those who believe in God, heaven would be an eternal post-earthly existence in the presence of God. Hell would be spending eternity in the absence of God.
I'm not sure how that works out, since I can't truly fathom what a state of being after life would actually be. The closest I can imagine is that Heaven would be a peaceful rest, while Hell might be non-existence. If they do exist, I'm not sure human minds would be able to comprehend them anyway.