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HTTP User Agent

A HTTP User Agent is a client that requests resources from a HTTP Server (that is also referred to by web server)
The expression user agent does not necessarliy mean that a human user is requesting the services of a HTTP server.

Message Cache

A user agent might cache a requested and obtained resource to save time and network bandwith the next time the same resource is needed.
Some HTTP headers control if and how long a resource is allowed to be cached.

Format

ProductName/Version (System and browser info) [Platform] [(Platform Details)] [extensions]
For example 
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0
sa User Agent Strings.

Changing user agent string

Firefox: about:config, then edit (or more likely create) general.useragent.override.
Firfox add-on User agent switcher
In Chrome, the user agent can be changed by opening Developer tools, then using the menu More tools -> Network conditions.

What's my user agent string

http://www.whoishostingthis.com/tools/user-agent/ shows the user agent string of the browser that accessed the page.

Determining the user agent

In PHP, the user agent string can be determined with $_SERVER['HTTP_USER_AGENT'].
In JavaScript, the user agent string is exposed in the browser object model through navigator.userAgent.
In Chrome, the user agent string that is sent to a web server can be queried with the special URL(?) chrome://version

See also

The .NET class Microsoft.PowerShell.Commands.PSUserAgent allows to construct a user agent string.
In the PowerShell invoke-webRequest cmdlet, a user agent string can be set with the -userAgent parameter.
HTTP
A user agent is supposed to identify itself using the User-Agent HTTP request header when requesting an HTTP service.
https://ifconfig.me/ua returns the requestors user agent

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