Citrix Metaframe for Windows sits on top of NT4 Terminal Server or W2K Server. There is also a version that sits on top of UNIX. The client can be run on Windows desktops, Macs, Unix/Linux, etc., machines. It is not an OS itself like Netware is.
You also have to have a Citrix license for each workstation connecting to it as well as a Windows Terminal Services license. (W2K Pro, XP Pro includes a free WTS license, you need to purchase a separate license for 95/98/Me/XpHome clients)