Document Outline window can be launched from the menu View -> Document Outline, or via short cut key Ctrl-Alt-T. The Document Outline window displays a nested, hierarchical tree of the elements and scripts on the page. It gives you a good overview of the page's content and its layout.
Double-clicking any element listed in the Document Outline window will highlight the corresponding markup for that element in the source view or the element display in the design view, depending on which view is currently open. This is very helpful in navigating a large, complex page.
More about Document Outline can be found here.
SDET, Visual Web Developer