If your Unix machine is suitably configured, hitting Ctrl + Alt + Backspace will immediately terminate the X server, or more accurately will cause the X server to immediately exit. This is an orderly exit from the server's perspective (it will do things like clean up the graphics state), but an abrupt one for clients; the server just closes their connections out of the blue. It turns out that the history of this feature is a bit more complicated than I thought.

Once upon a time, way back when, there was the X releases from the (MIT) X Consortium. These releases came with a canonical X server, with support for various Unix workstation hardware. For a long time, the only way to get this server to terminate abruptly was to send it a SIGINT or SIGQUIT signal. In X11R4, which I believe was released in 1989, IBM added a feature to the server drivers for their hardware (and thus to the X server that would run on their AIX workstations); if you hit Control, Alt, and Backspace, the server would act as if it had received a SIGINT signal and immediately exit.

(HP Apollo workstations also would immediately exit the X server if you hit the 'Abort/Exit' key that they had on their custom keyboard, but I consider this a different sort of thing since it's a dedicated key.)

In X11R5, released in 1991, two things happened. First, IBM actually documented this key sequence in server/ddx/ibm/README (previously it was only mentioned in the server's IBM-specific usage messages). Second, X386 was included in the release, and its X server hardware support also contained a Ctrl + Alt + Backspace 'terminate the server' feature. This feature was carried on into XFree86 and thus the version of the X server that everyone ran on Linux and the *BSDs. The X386 manpage documents it this way:

Ctrl+Alt+Backspace

Immediately kills the server -- no questions asked. (Can be disabled by specifying "dontzap" in the configuration file.)

I never used IBM workstations, so my first encounter with this was with X on either BSDi or Linux. I absorbed it as a PC X thing, one that was periodically handy for various reasons (for instance, if my session got into a weird state and I just wanted to yank the rug out from underneath it and start again).

For a long time, XFree86/Xorg defaulted to having this feature on. Various people thought that this was a bad idea, since it gives people an obscure gun to blow their foot off with, and eventually these people persuaded the Xorg people to change the default. In X11R7.5, released in October of 2009, Xorg changed things around so that C-A-B would default to off in a slightly tricky way and that you would normally use an XKB option to control this; see also the Xorg manpage.

(You can set this option by hand with setxkbmap, or your system may have an xorg.conf.d snippet that sets this up automatically. Note that running setxkbmap by hand normally merges your changes with the system settings; see its manpage.)

Sidebar: My understanding of how C-A-B works today

In the original X386 implementation (and the IBM one), the handling of C-A-B was directly hard-coded in the low level keyboard handling. If the code saw Backspace while Ctrl and Alt were down, it called the generic server code's GiveUp() function (which was also connected to SIGINT and SIGQUIT) and that was that.

In modern Xorg X with XKB, there's a level of indirection involved. The server has an abstracted Terminate_Server event (let's call it that) that triggers the X server exiting, and in order to use it you need to map some actual key combination to generate this event. The most convenient way to do this is through setxkbmap, provided that all you want is the Ctrl + Alt + Backspace combination, but apparently you can do this with xmodmap too and you'll probably have to do that if you want to invoke it through some other key combination.

The DontZap server setting still exists and still defaults to on, but what it controls today is whether or not the server will pay attention to a Terminate_Server event if you generate one. This is potentially useful if you want to not just disable C-A-B by default but also prevent people from enabling it at all.

I can see why the Xorg people did it this way and why it makes sense, but it does create extra intricacy.