bingbot is a web-crawling robot (type of internet bot), deployed by Microsoft to supply Bing (search engine). It collects documents from the web to build a searchable index for the Bing (search engine). It replaced msnbot as the main Bing Crawler on October 2010.
A typical user agent string for bingbot is "Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)". This appears in the web server logs to tell the webmaster who is requesting a file. Each webmaster is able to use the included agent identifier, "bingbot", to disallow or allow access to their site (by default access is allowed). If they don't want to grant access they can use the Robots Exclusion Standard to block it (relying on the assumed good behaviour of bingbot), or use other server specific means (relying on the web server to do the blocking).