This tutorial covers the various kinds of accounts available in Virtualmin.
Master Administrator -- this is the root user of the system. The root user has rights to manage any aspect of the server.
Reseller -- this user is created by the Master Administrator. They have rights to create Virtual Servers accounts for other users. Reseller accounts are a feature only available in Virtualmin Pro.
Virtual Server Owner -- a Virtual Server owner is the administrative user of a Virtual Server and any of it's Sub-Servers and Aliases. This user is created by either the Master Administrator or a Reseller.
Mail/FTP Users -- these users belong to a particular Virtual Server, and are setup to have Email access, FTP access, or both. These users are generally created by the Virtual Server Owner, though they can also be created by Resellers and the Master Administrator.