Some search engines are limited geographically. For example, only information on Web sites in Australia may be indexed. These search engines limit themselves to one subject, like biography, medicine, graphics, art, fishing, and so forth. These provide information from a set of reference works, such as an encyclopedia. Britannica, Bartleby and xrefer are examples.
Directories are categorized lists of sites picked out by human editors.
Directory databases are therefore much smaller than those of search engines. However, the fact that the sites are hand picked often means that you will find very high quality sites or articles in the results. Example directories are Yahoo, Look Smart, and Snap. Here are some suggestions about where to start a search. I've got a broad subject in mind, but don't know exactly what I'm interested in or how to look it up.