"Search engine" and "Web directory" are two different search services available to the Web community; although they are often mistakenly confused. Search engines have indices that are built up by robots or crawlers; whereas Web directories build up their indices through human editors. Many search engines and directories contain both a computer-generated index and a human generated index, and are referred to as hybrids. 

Google, Inktomi, AltaVista, AlltheWeb and the like are all forms of search engines. These search engines write programs known as robots, crawlers and/or spiders that have the following functions: (1) to locate Web pages, (2) to read the contents of the Web pages and (3) report its findings back to the search engine's indices or databases. Many search engines update their index either on a bi-monthly or monthly basis. When Web searchers use a search engine to locate Web sites that are relevant to the keyword search, they are searching the search engine's index. A search engine with a larger and more up-to-date index is a better representation of the information available in the Web.

Yahoo!, Open Directory Project (dmoz.org), Gipsy and the like are all forms of Web directories. These directories use human editors to review sites that are submitted for submission to the directory. Directories, unlike search engines, use a hierarchical tree structure to organize their database. Another common distinction is that a directory tends to list Web sites (root directory of a site or homepage) whereas a search engine will list Web pages (individual pages of a Web site). Due to the manual process of adding sites to a directory, directories often have to supplement their search results with a search engine partner to increase the relevancy of the produced search results. 

Search engines all have their own confidential algorithms that determine which Web pages are to be shown first. The algorithms assign weights to certain components or variables that it finds within a page. 

Web directories should be browsed through their hierarchical structure and not searched. Humans assign titles and descriptions that might not be within the source code of the page. Also, there is normally a submission fee to be added to a directory.