The xsession program is a session manager. It is normally executed by
your ~/.xinitrc (or ~/.xsession) script and controls your X Window
session. As soon as it is started, xsession launches a window manager
and some applications of your choice. At anytime during your session,
you may switch to another window manager or execute some other
applications from the xsession menus.

Examples may be found under /usr/share/doc/packages/xsession/examples.