A web directory is a directory on the World Wide Web, specializes in linking to other web sites and categorizing those links. The sites in web directory are listed in some order, most often alphabetic, and the users browse through them. The web directory is not search engine. It display lists of web pages based on category. The categorization is based on the whole web site rather.
The search function in a web directory is not the same with a search engine. Most web directories are edited by humans. And, the URLs are submitted by the site owners, not gathered automatically by spiders. No matter how clever spiders become, there is always a chance that a page will be in the wrong classified. To be indexed in search engine, you must wait the spiders to come to your site. But, once your site is in a web directory, it will increase your site visibility for spiders, and it will raise your chance to be indexed by search engine. To be in a web directory is good because it is a back link.
Web directories often allow site owners to directly submit their site. Sometimes a web directory accepts paid inclusion requests from websites that meet high standards.