**Do not participate in link schemes designed to increase your site's ranking or PageRank. In particular, avoid links to web spammers or "bad neighborhoods" on the web as your website may be affected adversely by those links. I don't get it, link exchange is key to SEO.... what do they consider a link scheme?
Avoid PTC and PTR sites. Autosurf and manual surf sites. Google has not only disabled adsense on sites using the above, but has banned users for using the above.