Subject-oriented information collections give you a way to find information by looking at a list of subject categories, clicking on the links of your area of interest, and then following links on the resulting pages of information to find your topic.
Subject-oriented information collections are sometimes called subject trees (or subject directories or search directories) because they "branch" out from their opening page to many subpages on many topics, which in turn branch out to more pages. These are different from keyword-oriented search engines which are interfaces to a database of information from Web pages examined by spiders.
Making a subject tree to cover all the information on the Web is an extremely difficult task because of the large number of Web documents.
However, there are Top Subject resources on the Web.
You can further explore the oldest Web-based subject tree, the WWW Virtual Library, About.com's guide-oriented method of describing Web subjects.
See Net Spaces reference chart for links to subject-searching resources for older information spaces such as Gopher as well as Usenet news.