Each symbol has four components (or “cells”), each of which references another object:
The symbol’s name.
The symbol’s current value as a variable.
The symbol’s function definition. It can also hold a symbol, a keymap, or a keyboard macro.
The symbol’s property list.
The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object.
The print name cell holds the string that is the name of a symbol.
Since symbols are represented textually by their names, it is
important not to have two symbols with the same name. The Lisp reader
ensures this: every time it reads a symbol, it looks for an existing
symbol with the specified name before it creates a new one. To get a
symbol’s name, use the function
symbol-name (see Creating Symbols).
The value cell holds a symbol’s value as a variable, which is what
you get if the symbol itself is evaluated as a Lisp expression.
See Variables, for details about how values are set and retrieved,
including complications such as local bindings and scoping
rules. Most symbols can have any Lisp object as a value, but certain
special symbols have values that cannot be changed; these include
nil and t, and any symbol whose name starts with
‘:’ (those are called keywords). See Constant Variables.
The function cell holds a symbol’s function definition. Often, we
refer to “the function
foo” when we really mean the function
stored in the function cell of
foo; we make the distinction
explicit only when necessary. Typically, the function cell is used to
hold a function (see Functions) or a macro (see Macros).
However, it can also be used to hold a symbol (see Function Indirection), keyboard macro (see Keyboard Macros), keymap
(see Keymaps), or autoload object (see Autoloading). To get
the contents of a symbol’s function cell, use the function
symbol-function (see Function Cells).
The property list cell normally should hold a correctly formatted
property list. To get a symbol’s property list, use the function
symbol-plist. See Symbol Properties.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol
void, nor the same as holding the
nil.) Examining a function or value cell that is void
results in an error, such as ‘Symbol's value as variable is void’.
Because each symbol has separate value and function cells, variables
names and function names do not conflict. For example, the symbol
buffer-file-name has a value (the name of the file being
visited in the current buffer) as well as a function definition (a
primitive function that returns the name of the file):
buffer-file-name ⇒ "/gnu/elisp/symbols.texi" (symbol-function 'buffer-file-name) ⇒ #<subr buffer-file-name>