In formatted text, Emacs distinguishes between two different kinds of
newlines, hard newlines and soft newlines. (You can enable
or disable this feature separately in any buffer with the command
Hard newlines are used to separate paragraphs, or items in a list, or
anywhere that there should always be a line break regardless of the
margins. The <RET> command (
newline) and C-o
open-line) insert hard newlines.
Soft newlines are used to make text fit between the margins. All the fill commands, including Auto Fill, insert soft newlines—and they delete only soft newlines.
Although hard and soft newlines look the same, it is important to bear the difference in mind. Do not use <RET> to break lines in the middle of filled paragraphs, or else you will get hard newlines that are barriers to further filling. Instead, let Auto Fill mode break lines, so that if the text or the margins change, Emacs can refill the lines properly. See Auto Fill.
On the other hand, in tables and lists, where the lines should always
remain as you type them, you can use <RET> to end lines. For these
lines, you may also want to set the justification style to
unfilled. See Format Justification.