Ok,
xkill will allow you to kill a program that has a GUI. Say, for instance, you're running Konqueror to view a webpage, and it all goes pear shaped. Konqueror seems to have crashed, but hasn't closed. You would run xkill and when you get the 'target/sights' mouse cursor, you click on the Konqueror display and it goes. You really should click on the 'taskbar' (I can't remember what it's called, but you know what I mean) immediately after running xkill. If this happens again, try doing the following:
Press <CTRL>+<ALT>+<BACKSPACE>. This should stop X (the graphical bit of Linux), but it will probably take you back to the login box.
