The DOM (Document Object Model)
Now before we move any further there are few things that you should know.
The browser has a definite components and it arranges all its components into a hierarchical tree structure. For example, the topmost component of browser is 'window'. After that the 'document' and so on. This hierarchical structure is called Document Object Model(DOM).
Finally here is a view of DOM objects. It will come in handy. Just have a closer look. No need to cram it.
'window' is the topmost component of DOM. Everything else is a sub-component of window. 'document' is sub-divided into forms, anchors, links and images.