Internet search engines (e.g. Google, AltaVista) help users find web pages on a given subject. The search engines maintain databases of web sites and use programs (often referred to as “spiders” or “robots”) to collect information, which is then indexed by the search engine. Similar services are provided by “directories” which maintain ordered lists of websites, e.g. Yahoo!
How Internet Search Engines Work
The good news about the Internet and its most visible component, the World Wide Web, is that there are hundreds of millions of pages available, waiting to present information on an amazing variety of topics.
When you need to know about a particular subject, how do you know which pages to read? If you're like most people, you visit an Internet search engine.
Internet search engines are special sites on the Web that are designed to help people find information stored on other sites. There are differences in the ways various search engines work, but they all perform three basic tasks:
• They search the Internet -- or select pieces of the Internet -- based on important words.
• They keep an index of the words they find, and where they find them.
• They allow users to look for words or combinations of words found in that index.
Early search engines held an index of a few hundred thousand pages and documents, and received maybe one or two thousand inquiries each day. Today, a top search engine will index hundreds of millions of pages, and respond to tens of millions of queries per day.
Before a search engine can tell you where a file or document is, it must be found. To find information on the hundreds of millions of Web pages that exist, a search engine employs special software robots, called spiders, to build lists of the words found on Web sites. When a spider is building its lists, the process is called Web crawling. (There are some disadvantages to calling part of the Internet the World Wide Web -- a large set of arachnid-centric names for tools is one of them.) In order to build and maintain a useful list of words, a search engine's spiders have to look at a lot of pages.
How does any spider start its travels over the Web? The usual starting points are lists of heavily used servers and very popular pages. The spider will begin with a popular site, indexing the words on its pages and following every link found within the site. In this way, the spidering system quickly begins to travel, spreading out across the most widely used portions of the Web.
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