How do search engines work:
Search engines allow the user to enter keywords that are run against a database 
(most often created automatically, by "spiders" or "robots"). 

Factors that influence results include: 
 • The size of the database
 • The frequency of updating
 • The search capabilities
 • Their search speed, the design of the search interface

What is the difference between a Search Engine and a Multi-Threaded Search Engine:
The growth in the number of search engines has led to the creation of "meta" search tools, often referred to as multi-threaded search engines.  Multi-threaded engines are very fast.  They search the databases of individual search engines simultaneously.

Yahoo is not a search engine, it is a subject directory.  That is, the database is created by man, and not randomly organized by the computer.  Directories tend to be smaller databases.

What is are Library Databases and Specialized Databases:
These are databases that focus on collecting Web sites or pages and arrange them by subject.   Specialized Databases concentrate within a defined subject area, geographical area, or type of resource.  Because these specialized search engines aim for depth of coverage within a single area, rather than breadth of coverage across subjects, they are often able to index documents that are not included even in the largest search engine databases.  For this reason, they offer a useful starting point for certain searches.

The "Invisible Web" is a large portion of the web which is out of reach for robots or spiders, therefore normally out of our reach.

  O t h e r  L i n k s 

For a really good Internet tutorial go to the University of South Carolina Beaufort Library's "Bare Bones"  Internet Tutorial (Lessons 1 to 5)
http://www.sc.edu/beaufort/library/bones.html

Another great place to go is "Finding Information on the Internet"  Teaching Library Internet Workshops University of California, Berkeley
http://www.lib.berkeley.edu/TeachingLib/Guides/Internet/FindInfo.html