The pioneer in this direction was a company named Goto, which changed its name to Overture prior to eventual acquisition by Yahoo! Goto was not, in the traditional sense, a search engine; rather, for every query term it accepted bids from companies who wanted their web page shown on the query . In response to the query , Goto would return the pages of all advertisers who bid for , ordered by their bids. Furthermore, when the user clicked on one of the returned results, the corresponding advertiser would make a payment to Goto (in the initial implementation, this payment equaled the advertiser's bid for ).
Several aspects of Goto's model are worth highlighting. First, a user typing the query into Goto's search interface was actively expressing an interest and intent related to the query . For instance, a user typing golf clubs is more likely to be imminently purchasing a set than one who is simply browsing news on golf. Second, Goto only got compensated when a user actually expressed interest in an advertisement - as evinced by the user clicking the advertisement. Taken together, these created a powerful mechanism by which to connect advertisers to consumers, quickly raising the annual revenues of Goto/Overture into hundreds of millions of dollars. This style of search engine came to be known variously as sponsored search or search advertising .
Given these two kinds of search engines - the ``pure'' search engines such as Google and Altavista, versus the sponsored search engines - the logical next step was to combine them into a single user experience. Current search engines follow precisely this model: they provide pure search results (generally known as algorithmic search results) as the primary response to a user's search, together with sponsored search results displayed separately and distinctively to the right of the algorithmic results. This is shown in Figure 19.6 . Retrieving sponsored search results and ranking them in response to a query has now become considerably more sophisticated than the simple Goto scheme; the process entails a blending of ideas from information retrieval and microeconomics, and is beyond the scope of this book. For advertisers, understanding how search engines do this ranking and how to allocate marketing campaign budgets to different keywords and to different sponsored search engines has become a profession known as search engine marketing (SEM).
The inherently economic motives underlying sponsored search give rise to attempts by some participants to subvert the system to their advantage. This can take many forms, one of which is known as click spam . There is currently no universally accepted definition of click spam. It refers (as the name suggests) to clicks on sponsored search results that are not from bona fide search users. For instance, a devious advertiser may attempt to exhaust the advertising budget of a competitor by clicking repeatedly (through the use of a robotic click generator) on that competitor's sponsored search advertisements. Search engines face the challenge of discerning which of the clicks they observe are part of a pattern of click spam, to avoid charging their advertiser clients for such clicks.