Mount

From Grokdoc

Please explain how you physically "mount" or "unmount," something; and when you do it. Do you have to "mount" something when you use it, and then "unmount" it again? After you have "mounted" something to use it, will it stay "mounted" forever, until you "unmount" it? Or will it automatically get "unmounted" when you switch the computer off, and you have to "remount" it again when you switch it on?