Please what is the differences in between?
Also if I am running only 2 or 3 websites in the same hosted VPS, which one is advised to be used?
Thanks 
You want to use the install.sh script which will install webmin and virtualmin for you.
You can find it under menubutton downloads. Start with the GPL version.
http://software.virtualmin.com/gpl/scripts/install.sh
Sorry if I am asking silly questions, but what are the differences in between so I can understand and know the differences?
Also which one is more light and less consuming resources?
Thanks
webmin is for managing the server
virtualmin is a module for webmin and is used to manage webhosting
usermin is a module for webmin and is used to manage the users email (webmail)
Cloudmin is used if you host VPS on one or more boxes.
Thanks a lot, it seems that I will go for it on centos and get step by step.
that sounds like a plan 
In addition to what ronald correctly said: The Virtualmin installer automatically installs Webmin and Usermin - along with all packages required (i.e. the software it configures). So it is highly recommended to use the installer on a freshly set up grade-a supported OS. Don’t install any hosting software packages manually (aside from the obvious SSH server and stuff) before you run the installer.
Ok, I will have a one click install for CentOS 6 32x and do an uprade, and update, then do the auto maticinstall