(This information comes from http://home.sprintmail.com/~debflanagan/main.html)

Subject directories or subject catalogues are sub-categorized overviews of internet resources. They contain only part of the resources of the entire Internet! However, all these sites have been selected by specialists and so they are likely to be better quality sites.

A subject directory contains an overview of subjects, sub-divided into often quite broad categories, such as art, recreation, science.

You can take a closer look at a particular category, for example, science. Here you are first referred to those Web pages that address the phenomenon of science in general. If you search for more specific information, 'science' is again sub-divided into the subcategories 'humanities,' 'physics,' 'exact sciences,' etc. If you choose ‘humanities,' this concept is in turn subdivided into information about sociology, economics, or psychology. Via economics, you finally find a reference to an interesting site in the field of, for example, monetary economics. As follows:

• science
  • physics
  • exact sciences
  • psychology
    • sociology
    • economics
    • psychology

As you can see, the references work hierarchically, from general to specific, from trunk to leaf, thus charting the whole scientific field.

In most subject directories, you can search by a list of categories, but also by filling in an input box. This creates the impression that you can also search the entire internet. In this case, however, you are still searching only that part of the internet selected by the specialists: in the subject directory.

The websites of subject directories have been selected and put into certain categories by experts. If you are looking for sites about Rembrandt, it is obvious that your search must start from a category like Arts & Humanities, followed by a subdivision like Painting and Masters/Artists. Sometimes, however, it is far from clear in which category you should start looking for a particular concept. In that case, it is a good idea to make use of the input box.

Subject directories cover only a small proportion of the entire internet.

Yahoo!, the biggest and most popular subject directory search engine, covers less than 5% of the Web. The Yahoo! people select sites and put them in a hierarchical system. There is no standard for such a system. Every subject directory uses its own, totally different, categorization.