Notebooks[ ] gives a list of notebooks currently open in the front end.
Notebooks[ ] returns a list of NotebookObject constructs.
Notebooks[fe] gives a list of notebooks open in a specific front end, specified by a FrontEndObject. The default is $FrontEnd.
See The Mathematica Book: Section 2.10.3.
See also: SelectedNotebook, EvaluationNotebook, NotebookOpen, Streams.