The X window system poses a serious security risk if it is not properly secured. If your X11 server (the screen, keyboard and mouse of any Unix system) is insecure, it will allow any other program on the internet to copy your screen's contents, capture your keystrokes, and sometimes forge keystrokes as if you typed them yourself.
Never use the command “xhost +”; this completely disables your display security!
Network access is blocked from outside our building to our X displays - X11 applications running offsite cannot send the display directly to a desktop at Tate Lab. In order to run such programs, it's neccessary to tunnel the X11 traffic through a secure shell session.
DISPLAY environment variable to open windows on your screen – if working from a graphical environment it should be set for you.DISPLAY variable at the remote system; ssh will have set it for you.
If you have problems, start by checking the value of DISPLAY at the remote system (using “printenv DISPLAY” or similar). Some servers don't switch on X11 tunnelling by default, in which case you have to use the “-Y” flag when connecting to them (eg “ssh -Y hostname”).