Terminal services let clients connect to a server via a network connection and run applications on the server while displaying the applications on the client device. The server and client communicate through a display protocol that uploads client input to the server and downloads graphical output to the client. Thus, the application appears to run on the client machine even though all processing except for rendering graphical instructions takes place on the terminal server. Because of the limited demands on the terminal server client machine, the machine doesn't need any hardware except a network connection, video support, and enough CPU and RAM power to render images on the screen.

Citrix developed a set of extensions for Windows NT 3.51 called MultiWin that makes the OS multiuser aware. MultiWin is the basis for Citrix WinFrame and several other versions of multiuser NT 3.51. Microsoft used MultiWin as the basis for its first multiuser version of NT Server 4.0—NT Server 4.0, Terminal Server Edition (WTS)—which the company released in July 1998. Terminal services based on the same technology are integral to Windows 2000 Server (Win2K Server). Although Citrix doesn't have a standalone version of multiuser NT 4.0, the company's MetaFrame product adds the necessary functionality to WTS. Windows 2000 (Win2K) and WTS use RDP to communicate with terminal clients; MetaFrame and WinFrame use the Independent Computing Architecture (ICA) display protocol.

For more information about terminal services and WTS, see the Web-exclusive article "An End to MetaFrame Dominance?" (http://www.winntmag.com/articles, InstantDoc ID 7041); Richard Harrison, "Putting Terminal Server to Work," April 1999; and Sean Daily, "A Case for Terminal Server," May 1999. For a review of WTS and MetaFrame 1.0, see "3 Ways to Be Thin," May 1999.