Google Adsense

Google Adsense is the most popular and widely used option. They serve contextual ads (image or text) based on your content, and pay when someone clicks on the ad (PPC - pay per click). How much you make on Adsense depends on several things. First, advertisers bid on keywords, so if you happen to write about a topic that has high paying keywords, then each click will be worth a lot more than another topic with low valued keywords. Also, people who came to your website or blog from search engines are more likely to click on these ads than people who are repeat visitors. It makes sense then to spread out your Adsense ads to provide easy access for search visitors.
You can get more information at Google's adsense website http://www.google.pt/ads/publisher/