The closest thing to a library catalogue for the world wide web is a search engine ' in fact, you might say that search engines constitute the central nervous system of the Internet itself. Search engines are the number 1 method for finding websites, businesses, products and services on the web.
When using a search engine, users enter keywords or keyword phrases which they think best describes what they are looking for. In its simplest form, a search engine will then return a list of web pages containing those keywords in some form.
Web search engines generally have paid listings and organic listings. Examples of organic search engines are Google and Yahoo. An example of a paid listing search engine is Overture. Search engines are generally broad, covering any type of website, but some are also topic-specific, region-specific, and even site-specfic.
Web search engines index most, but not all pages on the World Wide Web. Some require websites to register to be listed in them. The way search engines work is extremely technical and constantly evolving to improve accuracy. Put simply, they create indexes, or large databases of millions of web sites based on a range of rules that vary from search engine to search engine, including page titles, domain names, article and code or meta-tag keywords, and the number of links to and from other sites.