Similar like pay per click programs. You can place on your website some search boxes and if somebody search something then you are paid 0.01$-0.06$. This add some content to your site too:)). These days is more common share earnings so you are not paid flat rate by in %. How does it work? You put code on your website and when visitors clicks on that link you get paid 20%-80% from bidding amount which can be sometimes very high. Check each pay per click search engine for more details. Some of these affiliate programs have contextual advertising so if you have lot of text on your page than it automatically select some keywords and automatically create links and you are paid for clicks on them.