This tutorial covers Virtual Servers, Sub-Servers, and Aliases.
Virtual Server -- a Virtual Server is 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 a secondary domain name setup within a Virtual Server, with it's own domain name and set of email addresses and FTP accounts. A Virtual Server can have as many Sub-Servers as the Reseller or Master Administrator allow.
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.