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.