some space (virtual memory) on hard disk (called page file on windows and swap space on linux)usb drive as ReadyBoost in windows vista and above
I believe it's called the swap space. Google it and look for your self to make sure.
It's called the paging file. In most os's the actual file name is called 'pagefile.sys' and is a hidden OS file. You can modify the settings of the pagefile by right clicking on my computer ->...
virtual memory
Virtual memory stimulates additional ram