ZoneScreen is a tool for extending your desktop workspace using displays of network connected computers or portable devices like Pocket PC. Like VNC, ZoneScreen is able to mirror local desktop to another computer in the network. The main difference is that it also allows extend desktop, just like the case when you have second monitor connected.
All you need for ZoneScreen to work is network connection between your devices (your computers are both connected to the same network, or connected directly with cross-over cable or 1394 firewire cable, etc). In case you are using Poket PC, you may use connection provided by ActiveSync (using cradle cable, IR port, etc).
Extending functionality is supported for computers running Windows 2000, Windows Server 2003 and Windows XP (x86 and x64). Windows Server 2008, Windows Vista and Windows 7 support is limited.
ZoneScreen consists of two parts: kernel-mode ZoneScreen Virtual Display Driver which is visible to Windows just as normal video card with monitor attached to it, and user-mode ZoneScreen Wizard which is responsible for capturing image (running at server side), transmitting it over the network, and drawing it at another computer (running at client side). Machine with extended desktop called server. Server machine have Virtual Display Driver installed and ZoneScreen Wizard running in server mode. Another machine acts only as external display and called client. Client machine have only ZoneScreen Wizard running in client mode.
Both parts of ZoneScreen (driver and wizard) can run independently, though ZoneScreen Wizard performs better when using update API provided by ZoneScreen Virtual Display Driver. Otherwize it uses frame buffer polling which is CPU consuming and generally slow. This means that in case we are interested only in mirroring desktop image, we do not need to install Virtual Display Driver. And if we are extending desktop we could install ZoneScreen Virtual Display Driver and use any other image redirecting software on top of it. For example, modified TightVNC Server is capable of using ZoneScreen Driver update API.