Parallels Desktop 6.0 f/ Mac, 1-9u, IT PDFM6XL-01-IT ユーザーズマニュアル

Guest Operating System (Guest OS). An operating system installed inside your virtual machine. 
It can be Windows, Linux, Mac OS X, or any other supported operating system. 

Primary Operating System (Primary OS). The operating system that is loaded when you turn on 
your Mac – Mac OS X. 

Virtual Machine (VM). A computer emulated inside your Mac. A virtual machine has its own 
virtual hardware and requires an operating system. The installed operating system and its 
applications are isolated within the virtual machine and share physical hardware resources with 
your Mac.