This tutorial covers Virtual Servers, Sub-Servers, and Aliases.
Virtual Server
A Virtual Server is a server environment (which usually is a single hardware based computer machine) but virtualized. Instead of having multiple computers as multiple servers, you can use only one computer and simulate/virtualize multiple severs on one single machine. It is useful, if you need multiple servers for different tasks, that are not very recource intensive, to save on resources/hardware and space.
Virtualmin allows you, as an installation on your hardware server, to create those virtualized servers. This is implemented as an account in Virtualmin, that includes a website, email, and FTP access all associated with a domain name. Virtual Servers have an administrator account called the Virtual Server Owner. The Virtual Server Owner can create additional email addresses, ftp accounts, and websites, subject to the limits set by the Reseller or Master Administrator who created the Virtual Server.
Sub-Server
Generally created by a Virtual Server Owner, a Sub-Server is a secondary domain name setup within a Virtual Server, with its own domain name and set of email addresses and FTP accounts. A Virtual Server can have as many Sub-Servers as you need.
Alias
an alias is a way of making a domain name act exactly like another. If you have two domains, example.com and example.net, and you want both to have the same website, you would set example.net to be an alias of example.com