In the last decade search engines have become an important tool to find valuable content for a given query. Early search engines did rely on simply computing the similarities of query and page contents to find the most relevant results. Today, most engines incorporate some external relevance measure, like the page rank, to determine the correct ranking of web pages.
In this paper we will investigate the architecture of a search engine to understand how it works, discuss the history of Search Engines Marketing (SEM), the current business situation and the new features as the most famous search engines try to reach, supporting this paper with real case study comparing market share of Search Engine Marketing.

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