Don't think screens, think desktops. Each vncserver that launches (and binds to its own unique port) has its own separate desktop. Each runs its own distinct window manager application, and is not tied to a physical monitor or screen. Usually the default is TWM (minimalist window manager that takes up minimal resources), but you can also run GNOME or a host of other window managers if you want.
In OS X case, it's running the Finder.
If you log in as a user on Linux, and launch vncserver as that user, then that VNC session and desktop will be of that user.
One of the huge advantages is that the state is also maintained on the server. I've used it a LOT to install various Oracle software over the years. Fire up a VNC server session as an Oracle user, connect into it at the client site or office and start the installation. You can then disconnect, go home, do whatever, and connect in again remotely from another location (tunnelled over SSH usually), and the install will still be there running. It might have paused waiting for a dialogue to be answered, but it hasn't killed the install like logging out of the main desktop would have done.
You can also set the colour depth to be used by the client, thereby increasing performance over crappy SSH connections. 8-bit colour takes WAY less data than 32-bit, especially when installers have stupid meaningless animated gifs that run all the time.
In OS X case, it's running the Finder.
If you log in as a user on Linux, and launch vncserver as that user, then that VNC session and desktop will be of that user.
One of the huge advantages is that the state is also maintained on the server. I've used it a LOT to install various Oracle software over the years. Fire up a VNC server session as an Oracle user, connect into it at the client site or office and start the installation. You can then disconnect, go home, do whatever, and connect in again remotely from another location (tunnelled over SSH usually), and the install will still be there running. It might have paused waiting for a dialogue to be answered, but it hasn't killed the install like logging out of the main desktop would have done.
You can also set the colour depth to be used by the client, thereby increasing performance over crappy SSH connections. 8-bit colour takes WAY less data than 32-bit, especially when installers have stupid meaningless animated gifs that run all the time.