Pay per click (PPC) is an advertising technique used on websites, advertising networks, and search engines....PPC engines can be categorized in "Keyword", "Product", "Service" engines....
The largest seo companies in the pay per click industry are Yahoo! and Google. Google is not a pay per click search engine, but it does provide pay per click advertising in text ad boxes to the right of search results it delivers. It also delivers pay per click ads to other content sites.