The magic word is DISPLAY. In the X window system, a display
consists (simplified) of a keyboard, a mouse and a screen. A
display is managed by a server program, known as an X server.
The server serves displaying capabilities to other programs that
connect to it. A display is indicated with a name, for instance:
The display consists of a hostname (such as light.uni.verse and
localhost), a colon (:), and a sequence number (such as 0 and
4). The hostname of the display is the name of the computer
where the X server runs. An omitted hostname means the local
host. The sequence number is usually 0 -- it can be varied if
there are multiple displays connected to one computer. If you
ever come across a display indication with an extra .n attached
to it, that's the screen number. A display can actually have
multiple screens. Usually there's only one screen though, with
number n=0, so that's the default. Other forms of DISPLAY exist,
but the above will do for our purposes. For the technically
curious: hostname:D.S mean