A subject directory is a database of links to Internet resources. Links are grouped into subject categories, and information on a particular topic is usually found by browsing, although many subject directories are either linked to Internet search engines or offer an option to search their database by keywords.
Onlie subject directories may be described as being divided into two types, academic (or professional) and commercial. Academic or professional directories are compiled by librarians or subject specialists whose goal is to aid researchers by selecting only the best sites for each subject. As a result, they usually include fewer sites on a narrower range of subjects. The servers on which they are maintained are usually associated with colleges or universities, libraries, or professional societies.
The producers of commercial subject directories are in business to make money. Their revenue comes from advertisers, and this affects the subjects they cover and the websites they include. The inclusion of advertising may not necessarily indicate that a directory is a commercial enterprise, however. The producers of academic or professional directories may also accept advertising to help fund their work.
Commercial directories target a wider audience by covering more subjects, including many of purely popular appeal, such as sports, entertainment and hobbies. In most cases, it’s not clear who determines which sites to include, and some commercial directories have no selection criteria at all. Sites in the Yahoo! directory, for example, are recommended by the authors of the pages themselves. The Google directory is compiled by volunteer editors whose qualifications are not given.
The advantage of most commercial directories are the size and breadth of their databases and that most include some kind of search engine to facilitate retrieving pages on a subject.