The X Window System (or ``X'' for short) is a network-transparent window system which runs on many kinds of computers. Each workstation operates an X ``server'' which can display windows for various programs (``clients'') running on your computer. In addition, these windows may be displayed across the network from one workstation to another.
The xterm or rxvt clients create windows that allow access to the Unix prompt under X as if the window was a terminal. You can have multiple xterm or rxvt clients running at the same time; the default IMA setup opens one of these when you first login.
As configured at the IMA, by moving the pointer into a window with the mouse you make that window active. Input from the keyboard will be directed to that window until you move the mouse again. The window can be moved by clicking and dragging the gray bar at the top of the window. The rectangle on the left side of the terminal window is a scroll-bar; by dragging it using the mouse while holding down the middle mouse button, you can scroll the contents of the window up and down.
Text can be copied and pasted between windows with the mouse. Select text by dragging the mouse over it while holding down the left mouse button (turning it black). Move the mouse into another xterm or rxvt window. By clicking on the middle mouse button, you paste the selected text into that window.
Your DISPLAY environment variable tells X which X server to display a newly created window on. On login, this is the name of the workstation you're looking at followed by :0.0 (i.e., red.ima.umn.edu:0.0). When you use ssh to remotely login to another machine, the DISPLAY variable is usually automatically set to the machine you're logging in from.
Note: Some programs on our SGIs use GL graphics capabilities and cannot be displayed on an arbitrary X terminal. They can be displayed on other SGIs. OpenGL graphics can be displayed on other SGIs and on most of Linux workstations.