Most search engines use robots, often called 'bots, D2s, lookers, searchers, crawlers, spiders, worms or ants, to go to a web site, look at the pages and build an information database. Again, there are few standards but most use specific wording that they find in the coding of the web pages to record information about key words and the content of the page. Some look through all or most of the text on a page. Some do both. Yahoo, still uses a person to look at and enter a site's information into the database.
Some search engines use bulletin board like structures, often termed communities.
They all have the same purpose of helping you to find information faster. Some search engines only store information about a specific subject. Some, such as meta-engines, actually don't have databases of their own and don't do any research. They ask other search engines, more than one at a time, for the data you are looking to find. The CSGNetwork compilation of information about search engines , search sites and link sites is the largest on the Internet.