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 Programs), or a particular type of non-file buffer, such as a shell buffer (see Shell). 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). Minor modes are independent of one another, and of the selected major mode.