Subject directories are indexed and categorized by humans, not machines or software. Because of this their size tends to be smaller and they are not as up to date as some of the more powerful search engines. Users usually travel down a series of menus to search these sites, although keyword search is also available. They are not good for obscure topic searches. The biggest of the directories is Yahoo.
Search engines are huge databases of web sites and other internet resources.
There are more than 1200 search engines and they vary in speed, skill, depth of indexing, size of database, advanced search features and presentation of results. The spider returns periodically to each site to check for updates. Web authors also submit their website to different search engines for inclusion into the database. Each search engine has a proprietary software program that sifts through the huge database to find matches to a search. The software program then ranks the matches by what it perceives as their relevance.