In
X Window System terminology, a
display is an
X server. A
display can have
more than
one screen. The
format for a
display specifier string is like this:
host:dpynumber[.screen]
For example, tftv:0.0 is display 0, screen 0, on machine tftv. tftv:0 is just plain display 0, no screen specified. If no host is specified, it's localhost.
The $DISPLAY environment variable is the default display. You can specify a specific display with most programs by giving it a -display dpyname arg. GDK (and thus also GTK+) apps take --display, not -display.
A
display is done through a
socket opened with
Xlib call XOpenDisplay(char*), and
closed with XCloseDisplay(Display*).