Search Engines: Google, Yahoo!, Bing
Search Engines index (categorize) and find Web Pages. Search Engines delivers a page of results when Internet users type a phrase in a search box or tool bar.
Without Search Engines, the Internet would be more complex. Remembering the exact web addresses of every web site we'd visited would require complex storage systems within computers and mobile devices.
Search Engines simplify Internet searches by browsing most of the Internet with "bots" (short for robot), that constantly create and update maps of the Internet. So, when we type a word or phrase into the box in a box and hit return, most of the work has already been done for us. That's why the results are essentially instantaneous.
Search Engine Popularity, US Market Share, 2013:
Google - 66.5%
Bing - 17.3%
Yahoo - 12%
Others (AOL, Ask, etc.) - 4.2%
Source, Comscore, April 2013
Optimizing for all major search engines is fairly similar; given Google's dominant market share, SEO professionals tend to optimize based on Google results.