Search engines are programs that search documents for keywords and return a list of documents containing those keywords. Examples include Google and Yahoo. They work by using web crawlers to locate documents and indexers to create indexes of words in each document. Google provides many useful search features beyond basic keyword searches, including calculators, unit converters, clocks for different time zones, weather and flight status checks, and more specialized searches like site: searches. The document provides examples of how to use advanced search features on Google to refine searches by file format, domain, keyword locations, and excluding certain words.