How does a server detect a request made by Curl

Go to ifconfig.me , now run "curl ifconfig.me". You get different responses. How does the server know how a request was made?

EDIT: Here's a link that compliments the answer given below. It details the User-Agents curl uses. http://www.useragent开发者_如何学运维string.com/pages/curl/

The client passes information to the server in the request header. This includes, for example, the user agent string:

• http://en.wikipedia.org/wiki/HTTP_headers
• http://en.wikipedia.org/wiki/User_agent