Bingbot is a web-crawling robot (type of internet bot), deployed by Microsoft October 2010 to supply Bing.

[1] It collects documents from the web to build a searchable index for the Bing (search engine).

[2] 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.

[3] 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).