The term "search engine" is commonly applied to all manner of Web sites that assist an Internet user to find other Web sites. In fact, the term search engine should really only apply to those Web sites that use programs to automatically search for and index every Web page (and in many cases every word on every Web page) on the Net.
Search engines are also sometimes referred to as spiders or crawlers (or metacrawlers). Search engines are particularly useful for finding specific, hard-to-find information and, as such, serve as an invaluable tool for researchers. However, because they index indiscriminately, search engines commonly produce search results that include many Web pages that have little or nothing to do with what you were searching for.
Directories are completely different from search engines. Instead of being created automatically by a computer they are created manually by real live human beings and some of them apply pretty stringent quality control regimes. First, sites must be manually submitted by the Web site owner or Web designer who suggests which category or categories the Web site should be listed in. Then the site is checked out to make sure it exists and is competently constructed and that it fits the category or categories suggested. And then, if everything is in order, the Web site is registered. In most circumstances, directories provide better results than search engines and as a result the premier directory, Yahoo, is the world's most visited Web site.
And now there is a third type of resource to consider, hybrid search engines, search engines that also have an associated directory. Whether your search phrase initially returns a search engine listing, a directory listing, or a combination of the two sometimes depends on what the search phrase is as with Alta Vista, a very popular search engine that has just recently added a directory.
For more information: Search Engine Watch - Updated:
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