From Unconscious

Virtual Consoles

In some operating systems such as Linux and FreeBSD, a virtual console (VC, sometimes virtual terminal, VT) is a conceptual combination of the keyboard and the display for a user interface. The concrete combination is the console of the computer, where the user can switch between the virtual consoles to access multiple unrelated user interfaces. Usually in Linux, the first six virtual consoles provide a text terminal with a login prompt to a unix shell. The graphical X Window System starts in the seventh virtual console.

In Linux, the switching is performed with a key combination of Ctrl, Alt, and a function key. For example Ctrl-Alt-F1 to access the virtual console number 1, Ctrl-Alt-F2 to access the second, and so on. Ctrl-Alt-Left arrow changes to the previous virtual console and Ctrl-Alt-Right arrow to the next virtual console.

On Ubuntu, X11 will run on console no. 7, so Ctrl-Alt-F7 will take you back to X11. However, if you use ctrl-alt-backspace from X11, it will kill X11. Thus it will exit, and throw you back to the console it was started from (most likely no. 1). In this situation, Ctrl Alt F7 will not bring you back to X11, because it's not running anymore.