tightvncserver is a VNC server and an X11 server combine to one program. This means that tightvncserver accept connections from X11 clients, but there is no physical display. Instead, the “display” is directly relayed to VNC clients.
The fact that tightvncserver does not need or use any physical display has some interesting side effects. One side effect is that a single computer can run several instances of tightvncserver, differentiated by the port number (5900, 5901, 5902, etc.). Then VNC clients can connect to these “virtual” X11 desktops from other computers.
Recall that even Java in a web browser is sufficient to implement a VNC client. As a result, a possible cost effective computing environment (for schools and non-profit organizations) can use a single more power machine as the host of tightvncserver instances, while using many less expensive (possibly older) computers to act as VNC client programs. This method not only saves money, but it also simplifies the administration of computers.