PCs running Linux can be turned into ICA based client terminals using "free" (in price only) proprietary ICA client software from Citrix: Citrix Systems, Inc.. Unfortunately, MS requires that you purchase a license to cover the clients, even if the clients all run Linux. So if you want to save money on software costs by using Linux, you'll have to go all-Linux and use both Linux servers and clients using the free X-Window protocol.
The above is sometimes called "network computing" since the terminals and servers connect to each other over a network (such as the common TCP/IP based network used by both Linux and MS). Network computers may be somewhat different as described below.
Network computers (NCs)
These are neither true computers nor true terminals but are something in-between. One type of network computer (NC's) is a computer with a CPU but no hard Disk. The OS it needs to run is sent to it over a network. NCs are full-graphics and use the services of a server computer. They are a little different from terminals since some (or most) of the programs they run may execute on their own CPU chip. Running a browser was supposed to be one of their primary functions and thus Java code applets may be sent to them for execution. Many NCs support X Window so that one may use a Linux server to support it. Such a server may be called a "Linux Terminal Server". IBM called their NC a "NetStation" but now calls it "NetVista". They should work on Intranet type networks and NetVista can run the Linux OS.
Wintel came out with a "NetPC" which, unlike the above, is almost a PC computer. However, it has no removable disks so users can't install their own software or obtain copies of anything.
Thin clients and NCs under Linux
There is a "Linux Terminal Server Project" (LTSP or ltsp) to use Linux as a server for diskless thin clients. They use X Window and by default, applications run on the server. But with additional effort, one can set it up so that some or all applications run on the "terminal". See http://www.ltsp.org/.
"Terminal" in LTSP is actually a thin (or fat) client. This project's client can also run a telnet session and thus behave like a text-terminal. A software package named "lts" for the LTSP is available in the major Linux distributions.
It's claimed that if one has only a few "terminals", they will work without the ltsp software. But if one has many "terminals", ltsp software is needed. So use ltsp if what you want to do is to use old PCs, etc. as diskless thin clients. It works OK on systems with over 100 thin-client workstations.
Linux provides NFS (Network File System) so that if ordinary computers are connected to each other via a network, then a person on one computer can run programs on another computer. Such a program sends messages over the network so that it appears just like a program was being run by your local computer. But such a program is actually being run on another computer on the network. It works also with X Window so that one may see GUI images generated on another computer.
Linux also allows a computer to be diskless and boot over a network. See the "Terminal Server Project" above which has special software for this purpose. Network-boot-HOWTO gives an overview. Older documents are Diskless-HOWTO and Diskless-root-NFS-HOWTO. Thus using a diskless computer which runs NFS enables you to run programs on another computer (the server). This is just like using a NC (Network Computer). It's not really a NC but it's emulating a type of NC. It's also often called a "terminal" and in some sense it is.
Thus if you have an old PC with an ethernet card (NIC) you may be able to use it as a NC. One source of info on this is Thinclient-HOWTO. Even if your old PC doesn't have a NIC, you could
