Think of Google AdWords. How does it work? You enroll your site and pay Google so that when a user utilizes a particular key word, or string of words, your banner ad will appear not only on the results page of a search engine, but also on the websites that contain said string of words (provided that the sites avail of AdSense). As far as I know, personal sites and small businesses seldom use AdWords because of financial reasons, especially if you are up against big companies who can afford to pay more for ads.