WindowFrame is an option for Notebook which specifies the type of frame to draw around the window in which the notebook is displayed on the screen.
Typical possible settings are:
The details of how particular types of frames are rendered may differ from one computer system to another.
Settings for WindowFrame affect only the appearance of a window, and not any of its other characteristics.
See The Mathematica Book: Section 2.10.13.
See also: WindowTitle, WindowElements, WindowToolbars.