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
already in wide use for administration, but it is not that easy to
program a server yourself. This has been changed by LibVNCServer.

X.org already 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). The x11vnc binary (that allows you to export the window of a
real running X11 server) has been split off into its own package on
2007-07-16.