Unlike a library, the World Wide Web does not have a central index or uniform classification scheme that organises its contents systematically. In an attempt to provide ways to locate information on a particular topic, different searching tools or finding aids have been developed including search engines and directories. These tools are constantly evolving and many search sites now include both a search engine and a directory.

There are hundreds of these search tools in existence that all index the Web in different ways. They vary in size and possess different searching capabilities. No single search tool will always suit your needs exactly, as even the search engine that indexes the largest number of web resources only indexes about one third of the Web.

Search engines are finding tools. They are designed to scan the World Wide Web to search for details of sites and pages. This data is then stored in searchable indexes or databases.

You can search the contents of these databases by typing selected keywords in the text box provided on the search engine's home page. The program:

+ searches for occurrences of those words
+ retrieves any documents which match the terms from it's index
+ displays the results as a list of pages, ranked in order of relevance
(relevance is determined by different criteria for each program).

Basically search engines are made up of three parts. The first part is the "spider", also known as the "crawler", "worm" or "robot". The spider automatically visits www pages, examines them according to a preset criteria and then follows the links on those pages to other pages within those sites. The spider will return to sites periodically to examine their pages for changes. Information located by the spider is stored in the second part of the search engine, it's index. The third part of a search engine is the software that contains the program that searches through the index to retrieve and rank (once again according to a preset criteria) documents that match your search statement.

It is important to note that when you search using a search engine, you are searching only the contents of that search engine's index, not the contents of the entire World Wide Web.