What happens is that you put a minimum bid for a day (say $0.50). Whoever bids the highest amount for a day gets their advert displayed on your blog, you get $0.50 (minus PW's cut). However, if somebody bids $.60, they get the advert displayed.
It's only for a day though, to keep ad costs down, but you can put block bids on weeks or so.
And you have to approve all bids
