There are a number of things you can customize in X11. For example, you can customize your xterm window, set X11 application preferences, customize the X11 application and Dock menus, and specify which window manager to use.
To customize X11, you can create an .xinitrc script in your Home directory. A sample .xinitrc script is provided in /etc/X11/xinit/xinitrc.
Using the script as a starting point, you can specify which X11 -based applications to start when X11 is launched, including which window manager you'd like to use as your default. The default window manager for X11 is the Quartz window manager (or quartz-wm). The tab window manager (or twm) is also bundled with X11, but many other window managers are available. You can visit the following web sites to get instructions and binaries for a wide variety of window managers and DTEs:
If you're going to use your own .xinitrc file and want to use the Quartz window manager, make sure you start the Quartz window manager with the command:
Once you've installed X11, you'll probably want to install additional X11 applications, window managers, and perhaps other DTEs. (Even if you are using Apple's window manager, you can still run most binaries from other DTEs, such as GNOME and KDE, ...