Two other directories are Magellan and the Argus Clearinghouse. Use of these three tools illustrates the vastly different policies for selection criteria in different web directories. A major distinction between directories and indexes is the way their content is compiled. Indexes cast a broad net using artificially intelligent computer programs with descriptive names like crawler, bot, worm, spider, wanderer. These programs capture all information that meets their data-collection criteria.
Directories tend to be more discriminating.
Human editors normally sift through web documents and list those that meet a site's selection criteria. Directories tend to produce the most relevant results when you are searching for a general topic, but they may not be as comprehensive as indexes. One solution is the use of multi-threaded search tools that simultaneously search multiple directories and indexes. These are useful when the topic is obscure and you are not having luck with your search. These tools are also helpful when you want to find as much as you can with a single search statement and your search is not complex.