Difference between $(window).width() vs $(document).width()

From the documentation of width():

This method is also able to find the width of the window and document.

$(window).width();   // returns width of browser viewport
$(document).width(); // returns width of HTML document

Simple jsFiddle Demo

In the demo, I have set html { width: 1000px; }, which is bigger than the viewport.

The width of the body of your HTML page is a third value. $('body').width() can also differ from the other two (try body { margin: 100px; } for example).

You are correct. the window is the viewable area of the browser. The document is the actually body of the page. So your document could extend far beyond the window

Well, the window is the first thing that gets loaded into the browser. This window object has the majority of the properties like length, innerWidth, innerHeight, name, if it has been closed, its parents, and more.

What about the document object then?

The document object is your html document that will be loaded into the browser. The document actually gets loaded inside the window object and has properties available to it like title, URL, cookie, etc. What does this really mean? That means if you want to access a property for the window it is window.property, if it is document it is window.document.property which is also available in short as document.property.

So in conclusion the document could be smaller than the window.

FROM: http://eligeske.com/jquery/what-is-the-difference-between-document-and-window-objects-2/