Pay Per Click (PPC) is an Internet advertising model used on search engines, advertising networks, and content sites, such as blogs, in which advertisers pay their host only when their ad is clicked. With search engines, advertisers typically bid on keyword phrases relevant to their target market. Content sites commonly charge a fixed price per click rather than use a bidding system.

Websites that utilize PPC ads will display an advertisement when a keyword query matches an advertiser's keyword list, or when a content site displays relevant content. Such advertisements are called sponsored links or sponsored ads, and appear adjacent to or above organic results on search engine results pages, or anywhere a web developer chooses on a content site.

Although many PPC providers exist, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter are the three largest network operators, and all three operate under a bid-based model. Cost per click (CPC), varies depending on the search engine and the level of competition for a particular keyword.

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 web developers.

Determining Cost Per Click

There are two primary models for determining cost per click: flat-rate and bid-based. In both cases the advertiser must consider the potential value of a click from a given source. This value is based on the type of individual the advertiser is expecting to receive as a visitor to his or her website, and what the advertiser can gain from that visit, usually revenue, both in the short term as well as in the long term. As with other forms of advertising targeting is key, and factors that often play into PPC campaigns include the target's interest (often defined by a search term they have entered into a search engine, or the content of a page that they are browsing), intent (e.g. to purchase or not), location (for geo targeting), and the day and time that they are browsing.

Flat-Rate PPC

In the flat-rate model, the advertiser and publisher agree upon a fixed amount that will be paid for each click. In many cases the publisher has a rate card that lists the CPC within different areas of their website or network. These various amounts are often related to the content on pages, with content that generally attracts more valuable visitors having a higher CPC than content that attracts less valuable visitors. However, in many cases advertisers can negotiate lower rates, especially when commiting to a long-term or high-value contract.

The flat-rate model is particularly common to comparison shopping engines, which typically publish rate cards.. However, these rates are sometimes minimums and advertisers can pay more for greater visibility. These sites are usually neatly compartmentalized into product or service categories, which allows for a high degree of targeting by advertisers. In many cases, the entire core content of these sites is paid ads.