Not quite true.
You need to do a bit of setup.
1) Get an X server. If you're on Linux, you already have one, all graphics on Linux are routed via X. If you're on Windows, there are several available, mostly commercial but some are 30 day trials, or you can get a windows version of XFree (ie the same server used on linux).
I'll assume this is linux-to-linux.
2) Drop X security for the computer that will be connecting to you. In X, the program you run connects to the display server, NOT the other way around, counter to how you might think. By default, remote connections are disabled, for obvious reasons. Do:
to disable X security. Remember to renable it once you're done, with "xhost -".
3) ssh in to the machine where you want the program to run, like this:
ssh -C -X user@host
The -X is important, it enables X11 forwarding. The -C enables compression.
4) Run your X application. Start with xeyes, work up.
A few notes: this does not work well over high latency links, ie if the machine is on the opposite side of the planet, it's going to be slow. The X11 traffic will be encrypted and compressed, so a very slow machine may have trouble.
tightVNC is good, and has its place. X11 network transparency is most useful over LANs.