What does Usermin do?

Usermin is a web-based interface for web mail, password changing, mail filters, and much more, that seamlessly integrates with Webmin. It is designed for use by regular non-root users, and limits them to tasks that they would be able to perform if logged in via SSH or at the console. Those users have no access to system critical settings at all.

How does Usermin it help and how does it compare?

Most users of Usermin are sysadmins looking for a simple webmail interface. Unlike most other webmail solutions, it can be used to change passwords, read emails with no additional servers installed (like IMAP or POP3), setup users' configurations for forwarding, spam filtering and autoresponders.

Usermin can be understood as a simplified version of Webmin, without access to system critical settings or modules. All in all, Usermin is mostly a webmail client. While Virtualmin can be understood as a module of Webmin, Usermin is its own server and does not stand in a module like relationship to Webmin.