A lambda expression is a list that looks like this:
The first element of a lambda expression is always the symbol
lambda. This indicates that the list represents a function. The reason functions are defined to start with
lambda is so that other lists, intended for other uses, will not accidentally be valid as functions.
The second element is a list of symbols—the argument variable names (see Argument List). This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Local Variables.
The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Function Documentation.
The interactive declaration is a list of the form
(interactive code-string). This declares how to provide arguments if the function is used interactively. Functions with this declaration are called commands; they can be called using
M-x or bound to a key. Functions not intended to be called in this way should not have interactive declarations. See Defining Commands, for how to write an interactive declaration.
The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate"). The value returned by the function is the value returned by the last element of the body.