I think it has something to do with accessing a Unix machine from a Windows machine, but that could be wrong…
Is it some sort of standard, or is it an application?
If it’s a standard, what applications use it, and which would you recommend?
X Windows is the standard graphical user interface for Unix machines. It doesn’t have anything to do with Microsoft Windows[sup]TM[/sup]. It’s what allows you to use GUI apps and configuration tools on an Unix box so that a Unix box “looks” more like a Windows box, not just a command-line console.
Also it can have something to do with Windows in that you can buy a Microsoft Windows application that emulates X Windows so that you can use your PC as an X Windows client to run a graphical application on a UNIX server. Reflection X is one such emulator.
You don’t have to buy an X-windows server for MS-Windows. I use cygwin which includes a free X-server (as well as the usual GNU and so on Unix tools, all running under MS-Windows).
One important aspect of X-windows is that it us based on a network layer. So that you can run an application on a far off machine but have it’s window appear on your monitor. Extremely handy and only recently “discovered to be a useful thing” in the MS-Windows crowd. Think “thin client” and other nice things. E.g., you don’t have to install wiz-bang app that you just need once on your machine. Run it on a server that has it but redirect the window (including screen/mouse/keyboard) to your box. The OS’s/architecture don’t have to match at all. (Redmond shudders.)
Note especially that X-windows is completely unrelated to Unix. Run on Macs, Vaxen, OS/2, and on and on.
The standard X-servers are now being supplanted by other windowing environments such as Gnome and KDE.
X-windowing since v4.