A program that searches documents for specified keywords and returns a list of the documents where the keywords were found. Although search engine is really a general class of programs, the term is often used to specifically describe systems like Alta Vista and Excite that enable users to search for documents on the World Wide Web and USENET newsgroups.

Typically, a search engine works by sending out a spider to fetch as many documents as possible. Another program, called an indexer, then reads these documents and creates an index based on the words contained in each document. Each search engine uses a proprietary algorithm to create its indices such that, ideally, only meaningful results are returned for each query.

How do Search Engines work? 

Search engines help people find relevant information on the Internet. Major search engines maintain huge databases of web sites that users can search by typing in some text.
 

To compile their databases, search engines rely on computer programs called "robots" or, more specifically, "spiders." These programs "crawl" across the web by following links from site to site and indexing each site they visit. Each search engine uses its own set of criteria to decide what to include in its database. For example, some search engines index each page in a web site, while others index only the main page.

Also unique are the criteria each individual search engine uses to organize information for its users. Some list the results of a user's search according to which sites have the most links from other sites, a system known as link popularity. Other search engines prioritize results according to the summary information contained in sites' meta tags, and still others look for common themes used throughout a site. There are many other ways to organize results, and most search engines use a combination of several of them. 

Directories 

Directories are often confused with search engines, but actually they're completely different. Instead of using spiders to crawl the web, directories such as Yahoo! and Open Directory Project have real people who review and index their links. They also require web sites to adhere to rigid guidelines in order to be included in their indexes. As a result, directories' indexes tend to contain a comparatively small number of high-quality links.
The factors that influence search engine rankings simply don't apply to directory rankings. Instead, directory editors look at the quality of a site: its functionality, content and design. That means that webmasters hoping to see their sites listed on directories have to use very different strategies than for search engine placement.
 

Hybrid Search Engines

Hybrid search engines combine a directory with a search engine to give their visitors the most relevant and complete results. Today the top ten search sites are hybrids. For example, Yahoo! started out as a directory, but now it supplements its manually compiled listings with search results from Google, a search engine. On the other hand, Google uses Open Directory Project's directory to enrich its automatically generated listings.

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