Webmin is a GUI largely designed for a root user to manage a server.
Virtualmin is a Webmin module, which transforms Webmin into a system for managing a hosting environment. All the Webmin functionality is still there, but with the Virtualmin theme, it excels at managing domains, email, databases, and such – as would typically be found in a shared hosting environment.
Usermin is a web-based email client.
If you run the install.sh script found in the Downloads link above – you’ll end up with a system containing Webmin, Virtualmin, Usermin – and all the related tools configured for you (Apache, Postfix, BIND, MySQL, and so forth).