They differ from search engines which are organized like the index of a textbook. They are a good place to start a search for information on a well-defined broad topic. Human editors review websites and organize them under a series of headings and subheadings.
Content is organized for physicians and is peer-reviewed by physicians.
Free to use, but registration is required. You can compare results in different directories by cutting and pasting a search from one site to the other. A directory of Canadian websites. Contains a health sub-directory. The most popular Internet directory.