A web site search directory indexes a web site based on an independent description of a site. While directories perform many of the same functions of a web page search engine, their indexing format is different. The main difference is that directories do not spider your site to gather information about it. Instead they rely on a few text entries, typically a site title, domain name, and description, to determine which keywords describe your site.
Additionally, directories have hierarchical categories in which contain the description of your site along with others like yours. For example, a dry cleaning site may be listed under "business > retail > dry cleaning". Thus, users can find your listings by clicking through categories or using a search.