Google says you can't make pages simply to hang ads, "whether or not the page content is relevant." Many Web sites, including About.com, make money from ads. Google itself makes most of its money from advertising. What makes the difference between ad sponsored content and content for the sake of ads?
When you develop your site, your first thought should be about creating content, not ads. Avoid writing empty sentences for the sake of generating keywords, and avoid lengthy copy-and-pastes just to make more pages. Every page you publish should have a content-driven purpose.