A user agent is the internet name for a process.
It may be:
They are called user agent because they are acting on behalf of a user.
The most well-known user agent is a web browser but it can also be:
A GUI user agent such as a browser may:
When a software agent operates, it identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer.
In HTTP, SIP, and SMTP/NNTP protocols, the identifier:
For an email signing agent, the identifier is the AUID
The user agent string is another name for the user agent identifier value.