You can display multiple buffers in a single window, also called frame. Split buffers into two horizontal buffers with C-x 2, and with C-x 1 go back to displaying a single buffer into the frame.

Buffers can also be displayed next to each other, in columns. This can be handy when you need to compare two pieces of nearly identical text. Use C-x 3 to divide the current buffer into two buffers next to each other. Go back again to a single buffer with C-x 1.