A search engine is something that looks for specific information on the internet.  The results are sometimes referred to as hits.  The results can be delivered in many different formats like web pages, images, data and many other file types.  Some search engines are able to access information stored in databases or open directories.  Open directories are also on the internet and link to other websites.  They are not search engines though and don’t display lists of data based on certain words.  They list web sites by category and subcategory which is based on the entire web site instead of one page or a set of keywords.
There are many different search engines available today.  One of the more popular search engines is Google Search.  Google Search is owned by Google Inc. and is said to be the most used search engine on the internet, receiving more than 85% of the market share.  Each day Google Search receives several hundred million search requests through its different services.  One of the most common reasons for Google Search is to look for words or text on WebPages.  It has some 22 special features beyond the original word-search functionality. These include synonyms, weather forecasts, time zones, stock quotes, maps, earthquake data, movie showtimes, airports, home listings, and sports.
Another popular search engine is Yahoo!  Yahoo! Search is owned by Yahoo! Inc.  It is said to be 2nd largest search engine on the internet, receiving 6.3% of the market share.  That is quite a difference from Google’s market share.  In the beginning Yahoo! Search was a web directory of other websites organized in a hierarchy as opposed to a searchable index.  It wasn’t until the late 1990s that Yahoo! turned into a full-fledged portal with a search interface.  It would send queries to a searchable index of pages supplemented with its directory of sites. The results were presented to the user under the Yahoo! brand, however, none of the actual web crawling and data storage and retrieval was done by Yahoo! itself. In 2001 the searchable index was powered by Inktomi and then powered by Google until 2004, at which time Yahoo! Search became independent.  On July 29, 2009, Microsoft and Yahoo! announced a deal in which Bing would power Yahoo! Search. 
Bing Search engine is considered to be the fourth most used search engine on the internet.  It is previously known as Live Search, Windows Live Search and MSN Search.  It is marketed as a decision engine and was released from Microsoft in May of 2009.  Changes included the listing of search suggestions as users enter their queries and a list of related searches based on semantic technology.
Baidu is the fourth most used search engine on the internet and is owned by Baidu Inc.  This Chinese search engine is marketed as a search engine for websites, audio files, and images.  It provides 57 different search and community services which includes Baidu Baike which is an online collaboratively built encyclopedia and a a searchable keyword-based discussion forum.