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. A search engine is an information retrieval system designed to help find information stored on a computer system, such as on the World Wide Web, inside a corporate or proprietary network, or in a personal computer. The search engine allows one to ask for content meeting specific criteria (typically those containing a given word or phrase) and retrieves a list of items that match those criteria. This list is often sorted with respect to some measure of relevance of the results. Search engines use regularly updated indexes to operate quickly and efficiently.
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.
The vast majority of Internet users find new web sites by using a search engine. A position within the top 20 listings of a major search engine tremendously increases traffic to a web site.