Subject directories, unlike search engines, are created and maintained by human editors, not electronic spiders or robots. The editors review and select sites for inclusion in their directories on the basis of previously determined selection criteria. The resources they list are usually annotated. Directories tend to be smaller than search engine databases, typically indexing only the home page or top level pages of a site. When you initiate a keyword search of a directory's contents, the directory attempts to match your keywords and phrases with those in its written descriptions.
Subject directories come in assorted flavors.
There are general directories, academic directories, commercial directories, portals, and now, vortals. Portals are directories that have been created or taken over by commercial interests and then reconfigured to act as gateways to the web. These portal sites not only link to the popular subject categories, they also offer additional services such as email, current news, stock quotes, travel information and maps. Today, the line between subject directories and search engines is blurring. Most subject directories have added search engines to query their databases, while search engines are acquiring directories or creating their own.