There wouldn't be any special mention of how to kill a process in ubuntu
since processes can be killed in exactly the same way as they have for
the past 40 years in unix.

In a terminal, type "kill <process id>" if you know the process id - if
you don't know the process id, you can use the "killall" command and use
the process name - e.g. "killalll firefox".

As mentioned, you can also use the top command to view and selectively
kill processes.

If you're in a gui environment, there are several types of gui process
monitors, but generally, to kill a gui app one needs only to click the
terminate button on the upper right hand corner of the window.

But if you want to see a graphical representation, you can fire up the
process monitor. In gnome for instance, this is called "system monitor"
and it's in the system administraton menu.