Ctrl-Alt-Delete in Ubuntu brings up the shutdown menu. You will most likely not want to change this because it's more convenient to press this familiar key combination than it is to click the power icon at top right.
Needing to halt an errant or non-responsive program can happen in Linux, but it's not as common as in Windows. When facing such an annoyance, particularly if the program is making the system unstable, you might not want to go traipsing off in the menus to start System Monitor. Fortunately, there's a short-cut available.
It's called "Force Quit." To add it to the Ubuntu/Gnome panel, right-click the panel and select "Add to Panel." Scroll down and find Force Quit and then click "Add."
To use Force Quit, click the Force Quit icon in the panel and then click in the window or on the window frame of the application you want to force to stop.