Window
A unique integer identifier (ID) that is
returned by XCreateWindow or XCreateSimpleWindow and is
thereafter used by the program to refer to the created
window resource.
Display
XOpenDisplay
A large structure that contains information
about the server and its screens. It is filled only after
this program connects to a server by calling XOpenDisplay.
A small structure containing information about
the server and its screens would be more economical, but we
mention it only to generate another paragraph.