Search Engine
A type of software (Google being the best known) which creates indexes of databases and Web sites.
With "natural search", search engines use spiders which crawl the Web, gather the information, and then rank each Web page based on the titles of files, keywords, content relevancy, and incoming links.
With "paid search", advertisers pay to be placed at the top of the search results.
When you create a query in the search box of your browser, you are sending your request to the search engine and the matching results are returned to you on a search engine results page.