Pay per click (PPC) is an Internet advertising model used on search engines, advertising networks, and content websites, such as blogs, where advertisers only pay when a user actually clicks on an advertisement to visit the advertisers' website. With search engines, advertisers typically bid on keyword phrases relevant to their target market. When a user types a keyword query matching an advertiser's keyword list, or views a webpage with relevant content, the advertisements may be displayed. Such advertisements are called sponsored links or sponsored ads, and appear adjacent to or above the "natural" or organic results on search engine results pages, or anywhere a webmaster or blogger chooses on a content page. Content websites commonly charge a fixed price for a click rather than use a bidding mechanism.

Although many PPC providers exist, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter are the largest network operators as of 2007. Minimum prices per click, often referred to as costs per click (CPC), vary depending on the search engine and the level of competition for a particular phrase or keyword list—with some CPCs as low as US$0.01. Very popular search terms can cost much more on popular search engines. The PPC advertising model is open to abuse through click fraud, although Google and other search engines have implemented automated systems to guard against abusive clicks by competitors or corrupt webmasters.

Pay per click campaigns can be categorized into two major categories: sponsored match (or keyword) and content match. Sponsored match campaigns involve the display of advertisements on search engine results pages, whereas content match campaigns involve the display of advertisements on publisher websites, newsletters, and e-mails.

There are other types of pay per click programs that target product or service searches and product comparison sites. Search engine companies may participate in more than one category. PPC programs do not generate any revenue solely from Web traffic for websites that display the advertisements: Revenue is generated only when a user clicks on the advertisement itself.