NotebookObject[fe, id] is an object that represents an open notebook in the front end.
fe is a FrontEndObject which specifies the front end in which the notebook is open.
id is an integer that gives a unique serial number for this open notebook.
In StandardForm and OutputForm notebook objects are printed so as to indicate the current title of the window that would be used to display the notebook.
Functions such as NotebookPrint and NotebookClose take NotebookObject as their argument.
Within any open notebook, there is always a current selection. The current selection can be modified by applying functions such as SelectionMove to NotebookObject.
See The Mathematica Book: Section 2.10.3.
See also: NotebookSelection, NotebookOpen, Notebooks, SelectedNotebook.