When you enter a search term into a search engine, the robot generates a list of sites, with the most accurate 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. They catalog books and other materials and shelve them by universally accepted subjects. Magazine and journal articles are read and subjects assigned based on their content. In contrast, anyone can publish on the Internet. There is no one established system for organizing information, and there are numerous paths for finding what you need.