A window describes the portion of the screen that Emacs uses to display buffers. Every live window (see Basic Concepts of Emacs Windows) has one associated buffer, whose contents appear in that window. By contrast, a given buffer may appear in one window, no window, or several windows. Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.
Though many windows may exist simultaneously, at any time one window is designated the selected window (see Selecting Windows). This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer (see The Current Buffer), but this is not necessarily the case.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window) ⇒ #<window 1 on objects.texi>
See Windows, for a description of the functions that work on windows.