VNC is best known as a remote control application that is available for many OSes. As a remote control application, only one user can connect to a remote computer at a time and whatever the user does is visible on the display of the remote computer.

On Linux or Unix, however VNC can be set up to be multi-user, much in the same way that Terminal Services is.

(Apologies for these screen shots, I have not had time to produce better original ones)

This is the VNC client. The client is a tiny simple program that when copied on to a system does not even need a "setup" program.

When configured for multi-user, the user logs in through whatever desktop is in use on the remote server. They have their own desktop session that is invisible to other users on the system.

In this mode, the VNC server software intercepts X11 calls to create a virtual screen in memory and then sends that to the client using its own protocol.

This screen shot shows an example of  multiple sessions in use.

With VNC there is the ability to resume a session, unlike under pure X11. VNC does not have any facilities for bringing print jobs or audio to the local computer.

Unlike Microsoft Terminal Services or Citrix, VNC is free software and requires no expensive licenses just to connect to a server.

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