what is use of "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1) Gecko/20061010 Firefox/2.0" string in curl [duplicate]

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What is all the browser agent stuff?

What is use of Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1) Gecko/20061010开发者_高级运维 Firefox/2.0 string in curl? What is meaning of windows; u; windows NT 5.1; en-US; ry:1.8.1 and why is this string in used in curl with CURLOPT_USERAGENT?

Its not cURL, its HTTP, that allows a header User-Agent, that describes the user agent of the client.