Definition
A user agent is information regarding a device and or application that is sent as part of a request to another service or application.
Use Cases and Examples
A User-agent might look something like this Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0 which when broken down tells us the user is on Windows OS and using the Firefox browser as well as the version of each respectively.
The version can further be broken down to know if the device is mobile or desktop.
This information sometimes enables the service provider to better serve the requested content. For example, if the user agent shows that the content is being requested from a mobile device, the service provider might switch to provide content suitable for mobile devices.
User-agent data can also be used to provide analytics, so the application developers know which devices their customers are predominately using.
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