Internet Basics
A web search engine is a software system that is designed to search for information on the Internet. The information may be a mix of web pages, images, and other types of files. The search results are generally presented in a line of results often referred to as search engine results pages (SERPs). Some search engines also mine data available in databases or open directories. Unlike web directories, which are maintained only by human editors, search engines also maintain real-time information by running an algorithm on a web crawler. (Wikipedia)
How Search Engine Works
Web search engines work by storing information about many web pages, which they retrieve from the HTML markup of the pages. These pages are retrieved by a Web crawler (sometimes also known as a spider) — an automated Web crawler which follows every link on the site.
The search engine then analyzes the contents of each page to determine how it should be indexed (for example, words can be extracted from the titles, page content, headings, or special fields called meta tags). Data about web pages are stored in an index database for use in later queries. A query from a user can be a single word or a combination of words. The index helps find information relating to the query as quickly as possible.