A search engine is an information retrieval system which has stored a large amount of website’s data in its database. It can easily retrieve keywords, netsurfer information and quries by crawler in the minimum time.
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In 1991 a student of McGill University created the first 'search' facility in the University, for Communication with each others. After this invention the first automated link builder software package was released in 12 months.Search engine scan every documents within milliseconds in index of SE and gives the full detail of web sites with the help of Meta tags, Title, Description, contents and PR on the SERP. those policies & rules of SE which promote your site and carry on top listing index are called optimization. Other means performance of the websites can promote your ideal SEO theme.
Exclusions can be made by the use of robots.txt file. SE also analyzed to determine how it is in the site and what are the contents quality, copyright material, back links and relevancy. Different search engine have different policies & rules for searching like Google store all source page and refferd to chache for fastly retrieving. AltaVista store every word of every page and refferd to chach for holding, chach hold the actual text since it is one that was actually indexed. You can say it is very usefull way of searching when pages of the sites has been update then search term & condition are no longer in it. MSN has adopted Altavista methods although Yahoo runs like Google. Many others search engines use boolean opreators AND, OR and NOT. Some search engines provide an advanced feature of searching and define the distance between keywords are called proximity search.
Now a days search engines are going to commercial for revenue.they provide their services for advertising like Adword and Adsense. Adword is advertise from clint popularity therefor customer pay to search engine (pay per impresion PPI) for advertise and search engine pay to visitor against pay per click (PPC). Because SE use is as a searching platform therefor it work with consequently.