In formatted text, Emacs distinguishes between two different kinds of newlines, hard newlines and soft newlines.
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 section Auto Fill Mode.
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 section Justification in Formatted Text.