VNC is a set of programs using the RFB (Remote Frame Buffer) protocol.
They are designed to "export" a frame buffer via the network. It is in
wide use for administration.

X.org has a virtual Xvnc server which you can start as an own screen
(e.g. :1) and connect to with a VNC client (e.g. vncviewer from
tightvnc). This package contains the x11vnc binary that allows you to
export a screen of a running non-virtual X11 server. x11vnc has been
split off LibVNCServer on 2007-07-16.