Major and Minor Modes
Emacs contains many editing modes that alter its basic behavior in useful ways. These are divided into major modes and minor modes.
Major modes provide specialized facilities for working on a particular file type, such as a C source file (see Editing Programs), or a particular type of non-file buffer, such as a shell buffer (see Running Shell Commands from Emacs). Major modes are mutually exclusive; each buffer has one and only one major mode at any time.
Minor modes are optional features which you can turn on or off, not necessarily specific to a type of file or buffer. For example, Auto Fill mode is a minor mode in which SPC breaks lines between words as you type (see Auto Fill Mode). Minor modes are independent of one another, and of the selected major mode.