I want to know usb-hard-disk is how to be auto mounted?
 

My OS is ubuntu-desktop-9.10, when my usb-hard-disk is connected to pc, it will be mounted automatically and the icon will be showed at the desktop, now I want to know how these came about, what's the operational mechanism? and how I control it? for example, how can I disable the auto mount function? I'm beginner in learning Linux, thank you!

There is an Ubuntu wiki article on this.