A Web directory is completely different than a true search engine. It is an Internet site that contains information that have been examined and categorized in a directory.

A machine creates some directory sites, while one or more human beings create the vast majority.

Instead of sending out spiders, a Web directory uses people to review and index sites using a rigid set of criteria for deciding what sites to include and exclude from the directory. These people not only look at content, but also at the quality of the site and the user experience.

This is a manpower and time-consuming process and means that a Web directory will contain fewer sites than a search engine, although the sites and links are, arguably, of a better quality.

Another distinguishing feature is that a typical directory site allows you to browse through a tree of categories, sub-categories, sub-sub-categories, etc. With a search engine, you need to actually search, using either individual terms or search strings; you cannot simply click around.

Directories allow you to do just that—find things by clicking around and seeing what there is to see.

They contain large collections of categorized Internet sites and documents organized according to some intelligent and easy to navigate scheme.

About.com is a directory maintained by experts, called editors. About.com competitively hires expert editors, each of whom maintains the quality and content of each section of this directory.

Directories include About, MSN, and Yahoo.

Virtual Libraries

Numerous government agencies and universities house highly technical computerized catalog systems that are extremely large called Virtual Libraries. The Library of Congress hosts one of the largest in the United States. Originally, CERN created the 3W Virtual Library, in Switzerland, but it is now maintained by a consortium of institutions, including MIT.

If you search on the words “virtual libraries” you find a huge number of specialized cataloging systems for everything from microbiology to legal documents.

Meta-Search Engines

These are highly user-friendly Web sites that allow you to simultaneously send a single query to multiple search engines, directories, or specialized databases.

The Meta-Search Engine will then retrieve, combine, organize, and evaluate the results, often eliminating the duplicates, and ranking the reliability of the combined results. Some bring back all the results in one list; some let you see each source search engines results individually. Meta-search engines use a process known as fusion to select and combine results in their presentation to users. They usually only choose and use up to ten web sites from any of the other search engines they select for inclusion.

Dogpile, Mamma, ixQuik and travel search engines like Kayak and Mobissimo are metasearch engines.