When you enter a search term into a search engine, the robot generates a list of sites, with the best results appearing first. Every search engine's robot is different, and the way you should structure your search varies. Search engines are most useful when you know specifically what you want. Meta search engines allow you to enter your search term once, and have the query sent to many search engines simultaneously. Directories, on the other hand, are developed by human researchers, who review and categorize each site.
Directories are more selective than search engines.
Use a directory when you are looking for information about broad topics. Just as library materials have clues as to their accuracy, so do materials on the Internet. Cite the Internet resources you use. Websites are the obvious resources. But email is also a way to communicate with people, and some websites will even put you in touch with experts in various fields.