The other thread that mentioned Adsense made me think of this. I don't have a site launched yet but when I do I will definitely use Adsense ads. I have a couple of questions though...

1) How do you know how much you'll earn from your ads? Is there any way to tell what types of subjects pay the most?

2) I guess I don't understand why Google would actually pay us to display their ads. So they're giving some of the money away to us that advertisers are paying them for the ads?

Just want to make sure I understand everything.