Search Engine Robots or Web Crawlers

Most of the common users or visitors use different available search engines to search out the piece of information they required. But how this information is provided by search engines? Where from they have collected these information? Basically most of these search engines maintain their own database of information. These database includes the sites available in the webworld which ultimately maintain the detail web pages information for each available sites. Basically search engine do some background work by using robots to collect information and maintain the database. They make catalog of gathered information and then present it publicly or at-times for private use.