Web directories are Web sites that selectively catalog and categorize the broad range of sites available on the Web, usually including sites that are only likely to be of interest to a large number of users. Although they have quite a bit in common with Web search engines, web directory, such as the Yahoo! Directory, also differ tremendously from search engines. Web directories serve unique research purposes and in some cases may be the best starting point, even though their databases may include far less than 1 percent of what search engine databases cover.

The two most distinguishing characteristics of these tools (especially in contrast to Web search engines) are their selectivity and their classification (categorization) of sites. By selectivity, I mean that each site included in the free web directory is reviewed by a human being and included on the basis of some measure of quality. The underlying characteristic generally looked for is that the site must contain significant content and the content should be of interest to a fairly large number of people.

No matter how important, relevant, and useful they may be to you, commercial sites will probably not be found in the Yahoo! Directory unless these sites have paid a fee. For any site, chances of being included in the Yahoo! Directory in a timely fashion is greatly increased by the payment of a fee. In contrast, Open Directory Project does not accept any fees for inclusion.

A third characteristic of these tools is that typically only the main page of a site is indexed (in contrast to Web search engines, which may index all pages of a site). Web sites rather than Web pages are what is included in general Web directories. One impact of this distinction is that if a term is not on a site's main page, a directory will probably not identify that site as relevant for your search. Furthermore, directories are less likely to index every word even on the main page and may list and search only a brief description.

General Web directories typically organize sites into about a dozen broad categories, with each of those categories broken down into additional levels of hierarchy. This categorization can be the most important reason to go to a directory. It allows browsing through the levels of the classifications hierarchy and can provide valuable direction for a searcher who is not quite sure how to narrow down a broad topic. Different directories use different classification schemes, which may influence a user to choose one over another.