The way I understand it, some of the ads are run by outside programs like google. The site they are appearing on can create filters, more or less, that allow certain types of ads but not others, but they don't specifically choose the ads one by one. Things that the site might not actually want to promote occasionally slip through. The ads that appear are also tailored by google's algorithms (based on your search history and such), which are sometimes too comprehensive for their own good.
Other ads, like the Welch's juice ad, are paid for by companies to have a specific feature on the site.
Edited by: BITTERQUILL at: 1/13/2012 (10:36)