Search Engine Bots is short for Search Engine Robots and means a computer program which downloads all or a portion of your web site for the purpose of indexing within a Search Engine.

A Search Engine Bot can typically be identified by its User Agent string.

Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
YahooSeeker/1.0 (compatible; Mozilla 4.0; MSIE 5.5; http://help.yahoo.com/help/us/shop/merchant/)  
Baiduspider+(+http://help.baidu.jp/system/05.html)

The number of Search Engine Bots is rather large.

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