Web directories lead you to web sites by grouping them together according to subject.
Advantages of using a directory include:
Browsability. If you prefer to look for information by browsing, then directories are for you!
Selectivity. The web sites listed in directories have been hand-picked by real people according to certain guidelines. So you can expect that web sites located via a directory will at least be a cut above what you usually find on the Web.
Considerations about using directories to keep in mind:
Browsing by categories sounds easy, but it poses its own challenges. Sometimes it's hard to figure out what broad subject category might contain your topic. Also, the category names change from directory to directory, so you can't be sure that the term you used in one will work in another. Be persistent. If you can find a search box somewhere on the directory's home page, use a keyword search to pinpoint the area of the directory that contains your topic. Then you can continue browsing.
Obscure topics might not show up in a directory. In such a case, try a search engine instead.
Along the same lines, personal home pages will often not make it into a directory.
The most recently published web pages will not be indexed in directories, simply because it takes time for those editors to review new sites. If currency is important, use a search engine instead.