User-Agent
A User-Agent is a string in a request by which a browser or program tells the site what it is.
Every request has a User-Agent header: the browser name and version, the operating system, the device type. The site uses it to decide which version of the page to show.
Programs and code libraries have their own User-Agent, and it gives away an automated request right away. Many sites answer it with a refusal, the 403 code.
The User-Agent is only one of the traits by which a site recognizes a visitor. Together with the others they make up a browser fingerprint.
A proxy changes the IP address but not the User-Agent: your program sets it. It is important that they do not contradict each other, for example a mobile address and a desktop browser header.
FAQ
Do I need to change the User-Agent in a script?
A library's default header gives away an automated request right away. It is usually replaced with the header of a current browser.
Does a proxy change the User-Agent?
No. A proxy changes only the IP address; your program sends the headers.
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