Pay-per-click (PPC), also known as cost per click (CPC), is an internet advertising model used to direct traffic to websites, in which an advertiser pays a publisher (typically a website owner or a network of websites) when the ad is clicked. Pay-per-click is commonly associated with first-tier search engines (such as Google AdWords and Bing Ads). With search engines, advertisers typically bid on keyword phrases relevant to their target market. In contrast, content sites commonly charge a fixed price per click rather than use a bidding system. PPC "display" advertisements, also known as "banner" ads, are shown on web sites with related content that have agreed to show ads and are typically not pay-per-click advertising. Social networks such as Facebook and Twitter have also adopted pay-per-click as one of their advertising models.