Different Types of Search Engines..

                                 

The term "search engine" is often used generically to describe crawler-based search engines, human-powered directories, and hybrid search engines. These types of search engines gather their listings in different ways, through

1. crawler-based searches
2. human-powered directories
3. hybrid searches 

1. Crawler-based search engines 

Crawler-based search engines, such as Google (http://www.google.com), create their listings automatically. They "crawl" or "spider" the web, then people search through what they have found. If web pages are changed, crawler-based search engines eventually find these changes, and that can affect how those pages are listed. Page titles, body copy and other elements all play a role.

The life span of a typical web query normally lasts less than half a second, yet involves a number of different steps that must be completed before results can be delivered to a person seeking information. The following graphic (Figure 1) illustrates this life span (fromhttp://www.google.com/corporate/tech.html):

1. The web server sends the query to the index servers. The content inside the index servers is similar to the index in the back of a book - it tells which pages contain the words that match the query.
2. The query travels to the doc servers, which actually retrieve the stored documents. Snippets are generated to describe each search result.
3. The search results are returned to the user in a fraction of a second.

2. Human-powered directories

A human-powered directory, such as the Open Directory Project (http://www.dmoz.org/about.html) depends on humans for its listings. (Yahoo!, which used to be a directory, now gets its information from the use of crawlers.) A directory gets its information from submissions, which include a short description to the directory for the entire site, or from editors who write one for sites they review. A search looks for matches only in the descriptions submitted. Changing web pages, therefore, has no effect on how they are listed. Techniques that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed for free than a poor site.

3. Hybrid search engines

Hybrid search engines is the combination of crawler-based searches and human-powered directories.

Some of the Search Engines are.... 

Major Search Engines	News Search Engines	Multimedia Search	Job Search Engines
Google Yahoo Bing Vivisimo Clustering Engine Dogpile Altavista HotBot AlltheWeb AOL Ask.com Blekko Duck Duck Go Excite ExactSeek ePilot ICQ iWon Jayde Kanoodle Kosmix LookSmart Lycos Netscape Open Directory 7Search Teoma WiseNut Yebol	Bing News Google News Daylife MagPortal Newslookup Nexis (Lexis Nexis) Topix.net Yahoo! News	Bing Videos blinkx FindSounds Google Video Munax‘s PlayAudioVideo Picsearch Podscope ScienceStage Songza TV Genius Veveo TinEye Yahoo! Video YouTube	Bixee.com(India) CareerBuilder.com(USA) Craigslist(by city) Dice.com(USA) Eluta.ca(Canada) Incruit(Korea) Indeed.com(USA) LinkUp.com(USA) Monster.com(USA), (India) Naukri.com(India)