Too much information can be just as useless as no information if you have no way to sort through it.  A 'search engine', acting as a sort of index, allows you to type in a term and retrieve a list of sites containing the word or words you are searching.  The keywords are run against a database (most often created by "robots" that are called "spiders") which will return a list of literally thousands of web site pages.  Although the phrase is used generically, all search engines are not the same and the difference is in how the researcher's result listing is compiled.  Most search engines list results in terms of relevancy and frequency.  A set of rules are followed which involve location and frequency of keywords on the web page.  No one search engine covers the whole web in the same way, so if you don't find your answer with one . . . try another!!  

Search Engines
General Web pages

Subject Directories
Web pages and more

Directories of Searchable Databases
"Invisible" or "deep" web

Meta-Search Engines
Sound better than they work

(Created by University of California, Berkeley Library)