The emacs editor has a large collection of feature sets specific to a certain variety of text. The feature sets are called Major modes, and a buffer may have only one Major mode at a time.
A buffer's Major mode is private to the buffer and does not affect editing in any other buffer. If you switch to a new buffer having a different mode, rules for the new mode are immediately in effect. To avoid confusion, the name of a buffer's Major mode appears in the mode Line of any window viewing that buffer.
The three classes of Major modes are for
Editing human languages (for example, text, nroff, TeX)
Editing programming languages (for example, C, Fortran, Lisp)
Special purposes (for example, shell, mail, dired, ftp)
In addition, ...