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 section 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.