AdWords~Pay-Per-Click advertisements (PPC)

Advertisers specify the words that should trigger their ads and the maximum amount they are willing to pay per click. When a user searches Google's search engine on www.google.com, ads for relevant words are shown as "sponsored link" on the right side of the screen, and sometimes above the main search results.

The ordering of the paid listings depends on other advertisers' bids (pay for placement P4P) and the "quality score" of all ads shown for a given search. The quality score is calculated by historical click-through rates and the relevance of an advertiser's ad text, keyword, and landing page to the search, as determined by Google. The quality score is also used by Google to set the minimum bids for an advertiser's keywords.

The auction mechanism that determines the order of the ads has been called a "generalized second price" auction. It is a variation of the Vickrey auction.