Reaching across LAN segments, or through NATs and firewalls, was no picnic without resorting to VPN. The toughest thing about X11 used to be arranging for X11 clients to see your server.
They reach out to your server to tap your display, keyboard and mouse, but with far lower networking and compute overhead than full-screen remote desktop sessions require. X11 applications on remote hosts are clients. The X11.app that you run on your Mac is the server. In X11 parlance, the X11 server is the software that handles communications and renders client content.
Now we get to the most important step, which, once you understand the whole X11 client/server thing, is a walk in the park. In my previous two posts on the subject, I explained why you'd want to use X11 to drive a host remotely, and the basics of configuring your Mac to run OS X's X11 server and to use local X11 software.