While the differences between search engines and web directories are not always obvious, web directories generally list links by categories and subcategories, whereas search engines rely on keyword searches for results.  Most web directories also have search boxes, but a visitor could also follow the categories and subcategories to find links relevant to her interest.

Search engines often have robots that crawl and index sites on the web, whereas web directories list only links submitted to them.  Since web directories use a hierarchy of categories and subcategories to list sites, their pages are often crawled and indexed by the search engine robots.  This is why having your site listed on web directories is an especially effective tool to build your link popularity.  Your link is grouped with similar sites under a common category, making your listing more relevant in the eyes of the search engines. to the link popularity.