Pay per click (PPC) is an advertising model used on search engines, advertising networks, and content websites/blogs, where advertisers only pay when a user actually clicks on an ad to visit the advertiser's website. While many companies exist in this space, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter are the largest network operators as of 2007. Depending on the search engine, minimum prices per click start at US$0.01 (up to US$0.50), these prices are often referred to as Costs Per Click (CPC). Very popular search terms can cost much more on popular engines.