What is guest operating system?
The guest operating system in Oracle VirtualBox is the operating system that is running inside the virtual machine. Many guest operating systems are supported by Oracle VirtualBox, such as Windows 7 (32/64 bit), Windows 8 (32/64 bit), Windows Server 2008 (32/64-bit, also R2), Windows Server 2012 (also R2), Debian, SUSE, openSUSE, Mandriva, Fedora, Oracle linux, etc. To see a list of the supported guest operating systems, go to this link.
Just like with the operating system installation on a physical computer, you need to have an installer disc or an ISO image file of the operating system you would like to install. Note that a guest operating system does not have to be the same as the host operating system; for example, you can install a Linux distribution on a Windows operating system.