Buffers
All the commands you use to move text around within a file can be used to move text from one buffer to another. If you select the Buffers menu you can see the available buffers and pick which you want to visit.
If you type C-x C-b you will see something like
----------------------------------------------------------- MR Buffer Size Mode File -- ------ ---- ---- ---- . xcursors.1 303 Fundamental /tmp/xcursors.1 comp_sci 329 Fundamental /tmp/notes/comp_sci *scratch* 0 Lisp Interaction *Buffer List* 0 Buffer Menu -----------------------------------------------------------
This shows which mode each buffer has and what file, if any, it's associated with. To jump between buffers, use C-x b. By default you are invited to go to the last buffer you visited. You can save all the buffers by typing C-x s (rather than C-x C-s which just saves the current buffer).
Some types of buffers have their own special commands. For instance, the *Buffer List* buffer can't be written into but you can move the cursor around and select buffers for deletion (type d) or saving (type s) then type x for everything to happen. See the emacs Info section called Several Buffers for more info.