There are three major PPC search engines: Google Adwords, Yahoo! Search Marketing, and Microsoft adCenter (Bing). Search Networks: PPC advertisements show up as results in search engines. They are labeled as "Sponsored Links" and appear above and adjacent to organic search listings on search engines. Content Networks: PPC ads are placed in websites that have signed up to have ads appear on their site. For Content Networks, PPC ads are matched to the keywords in the site's content. Content Networks can be an excellent source of sales and leads. They can also get out of control unless they are created, optimized and tested by a highly skilled practitioner.