Virtual network computing (VNC) is a protocol that permits a computer be controled or viewed remotely. A VNC daemon is a program that connects to the desktop environment on one side, and yet connects to the network on the other side. It waits for an incoming request to connect. Once connected, a VNC daemon permits a VNC client (typically run on a different computer from the daemon) to interact with a graphical user interface (GUI) remotely.
VNC is interesting in a way because it is a daemon that is more commonly installed on workstations. This permits a worker to remotely access a computer at work and continue to work without having to install any special programs (other than a VNC client) on the home computer.
Note that Microsoft Windows can support VNC, but only on one desktop at a time (using a non-server license). In contrast, a Linux machine may provide any number of VNC environments (processing resources permitting). This means that the same Linux box can be accessed remotely by multiple users, each seeing his/her own GUI environment.