In order to make things simple we try to use the same USERNAME and PASSWORD for all virtual appliances. Yet there are some things you need to know.
This combination will get into most of the VirtualBoxImages VMs. You should change the password as soon as you start using your VM.
Ubuntu/Debian based distributions
Most of the Ubuntu/Debian distributions do not have the "root" user enabled. It is the "sudo" command that will allow you to perform most functions you would do as root. Another option is to execute the "sudo su" command followed by the password: adminuser.
RedHat/Fedora where root user is enabled
the root password: adminuser