Every browser offers the document it displays for manipulation via something called the Document Object Model, or DOM for short.  Older browsers supported their own DOMs, but there is a standard one, defined by the W3C, that is supported by most browsers these days.
The DOM models a document as a set of nodes, including element nodes, text nodes and attribute nodes.  Both elements and their text content are separate nodes.  Attribute nodes are the attributes of the elements.  If you want to see the document through the eyes of the browser, you can use a tool like the DOM Inspector of Firefox, which shows the document as a tree structure. 
Check out this video (.zip) narrated by Dori Smith that gives an overview of the Dom.
Here is a great resource link from the mozilla development center on the DOM.