Notebook[, , ... ] represents a notebook that can be manipulated by the Mathematica front end.
Notebook files contain explicit Notebook expressions written out in textual form.
You can manipulate open notebooks in the front end using standard commands in the front end, and using the options inspector.
Open notebooks in the front end are referred to in the kernel by NotebookObject constructs. You can use Options and SetOptions to look at and modify options for open notebooks.
See The Mathematica Book: Section 2.10.2.
See also: Cell.