NotebookOpen["name"] opens an existing notebook with the specified name, returning the corresponding notebook object.
NotebookOpen["name", options] opens a notebook using the options given.
NotebookOpen will usually cause a new notebook window to be opened on your screen.
NotebookOpen returns $Failed if it cannot open a notebook with the specified name.
NotebookOpen searches the directories specified by the NotebookPath global option for the front end.
With the option Visible->False set, NotebookOpen will return a NotebookObject, but will not cause a window to appear on your screen.
NotebookOpen initially sets the current selection to be before the first cell in the notebook.
See The Mathematica Book: Section 2.10.2 and Section 2.10.3.
See also: NotebookCreate, NotebookLocate, NotebookSelection, OpenRead, Get, SetSelectedNotebook.