The Google, Yahoo!, and Bing search engines insert advertising on their search results pages. The ads are designed to look similar to the search results, though different enough for readers to distinguish between ads and actual results. This is done with various differences in background, text, link colors, and/or placement on the page. However, the appearance of ads on all major search engines is so similar to genuine search results that a majority of search engine users cannot effectively distinguish between the two.
Remember when you used to rely solely on search engines for traffic? Remember when you worked on SEO and lived and died by your placement in Google? Were you #1? Assured success. Well, okay, maybe not assured. Success only came if the keywords were relevant to your site users, but it was the only real roadmap to generating site traffic and revenue.