A buffer contains your work; a window is a view of a buffer. Emacs calls its outer application window a frame. A frame can contain several Emacs windows, but only one is selected for ordinary commands. You can work with those views by name and spoken context without needing to picture their arrangement.
Switching buffers with C-x b changes the selected window’s contents. Selecting another window with C-x o moves to that view instead. The final character is lowercase o, for "other", not the digit zero. Emacsvox announces the selected buffer and mode; C-e m repeats that context and C-e l checks the current line. C-e C-w describes the window arrangement.
Two windows can show the same buffer at different positions. They are not independent document copies: edits in either change the same text, and the buffer has one mark. For two independent documents, select different buffers. The GNU Emacs Manual’s Multiple windows section explains this model in detail.