Web search engines work by storing information about many web pages, which they retrieve from the html itself. These pages are retrieved by a Web crawler (also known as a spider) — an automated Web browser which follows every link on the site.
The contents of each page are then analyzed to determine how it should be indexed (i.e, words are extracted from the titles, headings, or special fields called meta tags). Data about web pages are stored in an index database for use in later queries. The purpose of an index is to allow information to be found as quickly as possible.
When you are searching in Google/Bing/Yahoo with specific keywords, the engine examines its index and provides a listing of best-matching web pages according to its criteria, usually with a short summary containing the document's title and sometimes parts of the text.
Most search engines support the use of the boolean operators AND, OR and NOT to further specify the search query. Boolean operators are for literal searches that allow the user to refine and extend the terms of the search. The engine looks for the words or phrases exactly as entered. Some search engines provide an advanced feature called proximity search which allows users to define the distance between keywords. There is also concept-based searching where the research involves using statistical analysis on pages containing the words or phrases you search for. As well, natural language search allow the user to type a question in the same form one would ask it to a human. A site like this would be ask.com.
The usefulness of a search engine depends on the relevance of the result set it gives back. While there may be millions of web pages that include a particular word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines employ methods to rank the results to provide the "best" results first. How a search engine decides which pages are the best matches, and what order the results should be shown in, varies widely from one engine to another. The methods also change over time as Internet usage changes and new techniques evolve.
