In Keyword based PPC, advertisers bid on search terms (keyword phrase). When a user search for a particular keyword, the advertisers link display, the ordering of advertiser link depends on the amount of bid for the given keyword. So keywords are the heart of the PPC advertising. Many advertising companies offer software or services to help advertisers to choose best possible keywords for PPC advertising. Content Match, a service offered by Yahoo!, distributes the keyword ad to the search engines poster sites and/or publishers that have distribution agreement with the search engine company.
As of 2007, the following are notable PPC keyword search engines:
- Ask.com
- Baidu.com
- Google Adwords
- Look Smart
- Microsoft Adcenter
- MIVA.com
- Superpages.com