Important special-purpose commands

C-g

quits the current command. Generally useful for cancelling a M-x-style command or other multi-character command that you have started entering. When in doubt, use it.

C-x C-c

exits from Emacs. It prompts (in the echo area) if there are any buffers that have not been properly saved.

C-x u

undoes the effects of the last editing command. If repeated, it undoes each of the preceding commands in reverse order (there is a limit). This is an extremely important command; be sure to master it. This does not undo other kinds of commands; the cursor may end up at some rather odd places.

C-l

redraws the screen, and positions the current line to the center of the current window.