Indexes require you to develop expertise in mastering search language. One of the biggest problems with indexes is narrowing your search sufficiently to retrieve a manageable number of documents relevant to your topic. To stay with the book analogy, web directories are like a table of contents, which helps you locate major sections and subsections of a book. In their simplest form, web directories are merely catalogs of links to other web sites. They contain main subject headings and several levels of subheadings.
Some web directories are so vast that they have tools to search their own contents.
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.