You can select a major mode explicitly for the current buffer, but most of the time Emacs determines which mode to use based on the file name or on special text in the file.
Explicit selection of a new major mode is done with a M-x command.
From the name of a major mode, add -mode to get the name of a
command to select that mode. Thus, you can enter Lisp mode by executing
M-x lisp-mode.
When you visit a file, Emacs usually chooses the right major mode based
on the file's name. For example, files whose names end in `.c' are
edited in C mode. The correspondence between file names and major mode is
controlled by the variable auto-mode-alist. Its value is a list in
which each element has the form
(regexp . mode-function)
For example, one element normally found in the list has the form
("\\.c\\'" . c-mode), and it is responsible for selecting C mode
for files whose names end in `.c'. (Note that `\\' is needed in
Lisp syntax to include a `\' in the string, which is needed to
suppress the special meaning of `.' in regexps.) The only practical
way to change this variable is with Lisp code.
You can specify which major mode should be used for editing a certain file by a special sort of text in the first nonblank line of the file. The mode name should appear in this line both preceded and followed by `-*-'. Other text may appear on the line as well. For example,
;-*-Lisp-*-
tells Emacs to use Lisp mode. Such an explicit specification overrides any defaulting based on the file name. Note how the semicolon is used to make Lisp treat this line as a comment.
Another format of mode specification is
-*-Mode: modename;-*-
which allows you to specify local variables as well, like this:
-*- mode: modename; var: value; ... -*-
See section Local Variables in Files, for more information about this.
When a file's contents begin with `#!', it can serve as an executable shell command, which works by running an interpreter named on the file's first line. The rest of the file is used as input to the interpreter.
When you visit such a file in Emacs, if the file's name does not
specify a major mode, Emacs uses the interpreter name on the first line
to choose a mode. If the first line is the name of a recognized
interpreter program, such as `perl' or `tcl', Emacs uses a
mode appropriate for programs for that interpreter. The variable
interpreter-mode-alist specifies the correspondence between
interpreter program names and major modes.
When you visit a file that does not specify a major mode to use, or
when you create a new buffer with C-x b, the variable
default-major-mode specifies which major mode to use. Normally
its value is the symbol fundamental-mode, which specifies
Fundamental mode. If default-major-mode is nil, the major
mode is taken from the previously selected buffer.
If you change the major mode of a buffer, you can go back to the major
mode Emacs would choose automatically: use the command M-x
normal-mode to do this. This is the same function that
find-file calls to choose the major mode. It also processes
the file's local variables list if any.