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.