These volumes are 'mounted' or 'unmounted' on the Mac OS. This is one of the features that makes a Mac so powerful. You can clone one volume to another easily, whether it be physical or virtual. The hard drive icon you see on your desktop (probably Macintosh HD unless you renamed it) is a volume.
Volumes on a Mac are basically any physical or virtual disk that can be mounted permanently or temporarily. These are treated like a Volume of their own, but one that's contained in that file.
