On a text-only terminal, the Emacs display occupies the whole screen. On the X Window System, Emacs creates its own X windows to use. We use the term frame to mean an entire text-only screen or an entire X window used by Emacs. Emacs uses both kinds of frames in the same way to display your editing. Emacs normally starts out with just one frame, but you can create additional frames if you wish. See section Frames and X Windows.
When you start Emacs, the entire frame except for the last line is devoted to the text you are editing. This area is called window. The last line is a special echo area or minibuffer window where prompts appear and where you can enter responses. You can subdivide the large text window horizontally or vertically into multiple text windows, each of which can be used for a different file (see section Multiple Windows). In this manual, the word "window" always refers to the subdivisions of a frame within Emacs.
The window that the cursor is in is the selected window, in which editing takes place. Most Emacs commands implicitly apply to the text in the selected window (though mouse commands generally operate on whatever window you click them in, whether selected or not). The other windows display text for reference only, unless/until you select them. If you use multiple frames under the X Window System, then giving the input focus to a particular frame selects a window in that frame.
Each window's last line is a mode line which describes what is going on in that window. It appears in inverse video if the terminal supports that, and contains text that starts like `-----Emacs: something'. Its purpose is to indicate what buffer is being displayed above it in the window; what major and minor modes are in use; and whether the buffer contains unsaved changes.