What is Search Engine?

What is Search Engine?

What is Search Engine?

From a user standpoint a search engine is a web page that allows keyword or phrases to be entered and returns results based on the relevance of the documents it holds in its index. Relevance is determined by a set of automated routines referred to as algorithms. Search Engines index many billions of pages of Internet content ranging from web pages, images, Adobe Portable Document Format (PDF) and Microsoft Word documents through to music and video clips. It would be impossible for a search engine to explore all the data it indexes in real time. Instead it runs a number of automated programs called robots (also called bots, crawlers or spiders). These programs explore the content referenced in the search engine's index. Starting from the home page of a website and by following the links it funds, a robot can eventually explore all the linked page in the site and even discover new websites through outbound links. This process is called spidering. Each document that the robot discovers is ranked based on the search engine's algorithms. This provides a concise picture of the content of the page and it is this information that is used when a search is performed.

The search engine builds up its index either by having webmasters submit their sites directly via free or paid submissions pages or through discovering new sites during the spidering process described above. Obviously if a search engine doesn't know about a page it won't be found by it's users. Yahoo! say that they index around 80% of the Web although it is difficult to know how accurate this statement is. Google claims to have over 8 billion pages in their index. In order to keep the index fresh robots must periodically revisit a site. The amount of time between visits depends on the resources available to the search engine and how up-to-date the search engine want to keep a particular page. Some of these periodic visits, such as those prior to the Google dance, generate much excitement from search engine experts.

Examples of general purpose Internet search are Google, Yahoo and MSN Search. Most search engines offer advanced search engine option that allow users to narrow down their search criteria, for example to find recently updated web pages or pages in a certain language. Some popular search engine use indexes from other engines, at the time of writing AOL search is powered by Google's technology. There are also a number of specialty search engines, an example being Google News which indexes news stories from around the world. Google is currently the most popular Internet search engine. Users perform over 200 million searches per day on Google and it is estimated that it handles between a half and three quarters off all searches.

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