Each buffer can be in two kinds of mode: a major mode, such as Lisp mode, or Fundamental mode (which is the ordinary text processing mode); and a minor mode, such as Abbrev mode or Auto-Fill mode. A buffer always has precisely one major mode associated with it, but minor modes are optional. Any number of minor modes can be associated with a buffer.
The major modes govern how certain commands behave. For example, the concept of indentation is radically different between Lisp mode and Fundamental mode. As another example, a Directory mode buffer (which is essentially read-only) lists files and allows you to operate on them with simple keystrokes like E for edit and D for delete. The file listing is updated automatically to reflect any changes.
When a file is loaded into a new buffer, the default mode of that buffer can be determined by the file name. For example, a buffer into which a file name that has a .lisp suffix is loaded defaults to Lisp mode.
The minor modes determine whether or not certain actions take place. For example, when Auto-Fill mode is on, lines are automatically broken at the right hand margin, as the text is being typed, when the line length exceeds a pre-defined limit. Normally the newline has to be entered manually at the end of each line.
Editor User Guide (Macintosh version) - 01 Dec 2021 19:35:08