Indexes are databases of URLs that have been collected from around the WWW. Essentially they are registries of Web sites based on descriptions by Webmasters or written by the directory's staff.

One of the biggest subject organized indexes is Yahoo. It is very useful to find good collections of resources for a topic.
 

II. SEARCH ENGINES

Web search engines attempt to create a detailed record of the Web using automated software agents - nicknamed spiders - that crawl from URL to URL. They visit every site in the public areas available on the Web and record the addresses. 

Each of these search engines offers essentially the same service: You log onto the page with a browser, type a query into a box, and the programme returns a list of clickable links with a short description. Special software is not needed. 
However, the search engines vary significantly in the volume of information contained in their databases. None of the search engines can cover all the websites available. Moreover, differences in results emerge when searches become more complex or specific. If you cannot find what you are looking for with one search engine, try another one. However, keep in mind that not everything can be found on the Internet. For some questions the Internet might just not be the right source to use. (See also: Search Tips and Tricks)

In addition to individual search tools or engines you can also find Meta Search Engines on the Web. A Meta-search engine differs from an "ordinary" search engine or search tool (such as AltaVista or Lycos) in the sense that it transmits your search simultaneously to most of the popular search engines and their databases of webpages. The result you get is a compilation of links containing matching sites from all the search engines queried. Meta-search engines do not own any databases but use and deliver the databases and searching programmes of each of the individual search tools they query.
Using a Meta-search engine can often save time because you retrieve results from different search engines all at once. However, if you submit a search that one or more of the individual search engines queried cannot "understand" you will not receive results from those engines or sometimes just get "garbled results". It is important to know how to search the individual search engines in order to be able to judge the returned search results from Meta-search engines. Also, often it is more useful to use an individual search engine or tool in order not to be overwhelmed by the number of results.

For a more in-depth discussion of Meta-search engines please see: Meta Search Engines (University of California, Berkeley)

There are numerous Web search engines available for free on the Internet.