From Seo Wiki - Search Engine Optimization and Programming Languages
In-text advertising is a form of contextual advertising where specific keywords within the text of a web-page are matched with advertising and/or related information units.
Although contextual advertising in general refers to the inclusion of advertisements adjacent to relevant online context (e.g., Google AdSense), in-text advertising places hyperlinks directly into the text of the webpage. Most in-text advertising has the following characteristics:
- The text associated with an advertisement is identified by a double-underline to differentiate it from regular hyperlinks.
- An in-page bubble containing advertising content appears when the cursor is positioned over the corresponding text.
- When the mouse is moved away from the double-underline hyperlink, the bubble disappears.
- If the reader is interested in learning more about the product being offered, he can simply click on the ad bubble to go to the advertiser's site.
- Some examples of in-text ad providers include Vibrant, HOTWords, Infolinks and Kontera
In text advertising works on a cost per click (CPC) model, which means that each time a website visitor clicks on an In-Text ad, the websites owner gets paid by the advertiser.
In-text advertising can supplement other methods of contextual advertising since it monetizes the website content itself without taking any extra space of the web-page. The integration process of in-text ads usually requires the insertion of script code into the web-pages' html. After that, the entire process is intelligently automated. The advantages of this type of advertising have been discussed on the Online Marketing professional blog - Online Siesta 
The use of this type of advertising in news and journalism websites has been criticized by journalism ethics counselors as "ethically problematic at the least and potentially quite corrosive of journalistic quality and credibility." However, publishers such as the Indianapolis Star who use in-text advertising have reported that despite early objections by some readers, such complaints have "tapered off".