X is based on the client-server approach: the client runs the application while the server displays it on the local display (note that X uses a different definition for ``client" and ``server"; the system running the application is termed the ``client" while the system displaying the application is the ``server").
Most utilities based on the X window system are very easy to customize. They can usually be resized, moved, put on top of other windows, and pushed behind other windows interactively. Their appearance can also be altered significantly with the help of resources, through the use of the .Xdefaults file, described in one of the sections below.
Some utilities/programs based on the X Window system include a terminal emulator (xterm), window managers (4Dwm, twm), a news reader (xrn), access control programs (xhost, xauth), a load monitor (xload), a clock (xclock), a calculator (xcalc), utilities for listing information about fonts, windows, and displays (xlsfonts, xfontsel, xlswins, xlsclients, xdpinfo, xprop), screen image manipulation utilities (xwd, xwud, xpr, xmag), 2-D graphics programs (xmgr, gnuplot), a public domain retrieval system (xarchie), a PostScript file previewer (ghostview), and many, many more utilities, many of which are in the public domain.