Lisp functions are executable code, just like functions in other
programming languages. In Lisp, unlike most languages, functions are
also Lisp objects. A non-compiled function in Lisp is a lambda
expression: that is, a list whose first element is the symbol
lambda (see Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, you can construct or obtain
a function object at run time and then call it with the primitive
apply. See Calling Functions.