PPC
Pay per click (PPC) (also called Cost per click) is an Internet advertising model used to direct traffic to websites. With search engines, advertisers typically bid on keyword phrases relevant to their target market. This approach differs from the "pay per impression" methods used in television and newspaper advertising.
Advertisers select the words that should trigger their ads and the maximum amount they will pay per click. When a user searches on Google, ads (also known as "creatives" within Google) for relevant words appear as "sponsored links" on the right side of the screen, and sometimes above the main search results. Click-through rates (CTR) for the ads are about 8% for the first ad, 5% for the second one, and 2.5% for the third one. Search results can return from 0 to 12 ads.
The ordering of the paid-for listings depends on other advertisers' bids (PPC) and the "quality score" of all ads shown for a given search. The search-engine system calculates the quality score on the basis of historical click-through rates, relevance of an advertiser's ad text and keywords, an advertiser's account history, and other relevance factors as determined by Google.