Pay per click ads may also appear on content network websites. In this case, ad networks such as Google AdSense and Yahoo! Publisher Network attempt to provide ads that are relevant to the content of the page where they appear, and no search function is involved.
While many companies exist in this space, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter are the largest network operators as of 2007. Minimum prices per click, often referred to as Costs Per Click (CPC), vary depending on the search engine, with some as low at $0.01. Very popular search terms can cost much more on popular engines. Arguably this advertising model may be open to abuse through click fraud, although Google and other search engines have implemented automated systems to guard against this.[1]