The Window interface represents a window containing a DOM document; the document property points to the DOM document loaded in that window. A window for a given document can be obtained using the document.defaultView property.
There are no official standards for the Browser Object Model (BOM).
The window object is supported by all browsers. It represents the browser's window.
Global variables are properties of the window object.
Global functions are methods of the window object.
Even the document object (of the HTML DOM) is a property of the window object:
window.document.getElementById("header"); is equal to document.getElementById("header");