A window displays the contents of a single buffer. A buffer need not be displayed in a window, however.
At the bottom of each window is a ModeLine that describes the current buffer status.
Here is a screenshot showing a frame with three windows:
Emacs may be thought of as a tiling window manager: the screen is organized into mutually non-overlapping windows, as opposed to the more popular approach of “floating” windows.
Commonly used window commands include these:
C-x 0 delete the current window C-x 1 maximize the current window C-x 2 split the current window horizontally C-x 3 split the current window vertically C-x ^ make selected window taller C-x } make selected window wider C-x - shrink window if larger than buffer
See also WindowsAndFrames.