Large databases of web pages assembled by machines called “spiders or crawlers” based on specified criteria.
Web contents created & maintained by humans using specific criteria. Sometimes called general directories. Commercially sponsored subject directories are called portals (i.e. MSN, Excite, etc.)
Search engines specifically designed for a younger audience.
Web resources arranged by subject & recommended by specialist (usually librarians). Like a special kind of subject directory.
Designed to help find images on the web.