delete-backward-char).
backward-kill-word).
backward-kill-sentence).
The <DEL> character (delete-backward-char) is the most
important correction command. It deletes the character before point.
When <DEL> follows a self-inserting character command, you can think
of it as canceling that command. However, avoid the mistake of thinking
of <DEL> as a general way to cancel a command!
When your mistake is longer than a couple of characters, it might be more convenient to use M-<DEL> or C-x <DEL>. M-<DEL> kills back to the start of the last word, and C-x <DEL> kills back to the start of the last sentence. C-x <DEL> is particularly useful when you change your mind about the phrasing of the text you are writing. M-<DEL> and C-x <DEL> save the killed text for C-y and M-y to retrieve. See Yanking.
M-<DEL> is often useful even when you have typed only a few characters wrong, if you know you are confused in your typing and aren't sure exactly what you typed. At such a time, you cannot correct with <DEL> except by looking at the screen to see what you did. Often it requires less thought to kill the whole word and start again.