System hang means the system is unresponsive to keyboard inputs.
A hang may be temporary or permanent.
Temporary hang: If it is a temporary hang the system resolves itself.
Slow hardware is a kind of temporary hang.
Permanent hang: If it is a permanent hang it requires manual intervention.
It may be due to the hardware or software logic error.
'End Task' button on the windows task manager is a mechanism for terminating hang processes.
The computer's graphical user interface no longer responds to the user inputs like typing on the keyboard or moving the mouse is a typical example for hang.