The trick is to figure out which category will contain what you're looking for. Directories have less sites available than a search engine and are usually more useful when you're looking for a general topic that's easily described. Search engines like Northern Light or Altavista can also help you find what you're after. Search engines are powered by software instead of human categorizers. The engines robotically look through millions of Web pages and create a database of the results, which you search by keyword.
Search engines can find more than directories, but can also be less precise.
A meta-search engine like ixquick or Metacrawler searches several engines at once and groups the results. These results are usually broken down in a list arranged by the name of the original search engine that found them. The meta-search helps to remove duplicate sites but misses some of the special features you get by using the search engine directly. Subject guides like Berkeley Public Library's Librarian's Index to the Internet or the University of Kentucky's World Wide Web Resources are an additional way to help you narrow down the resources available on the Web. These are created by information professionals and arranged by academic discipline or general popular subject.