Lisp programs are run through an interpreter. Once one expression is entered, the interpreter evaluates it according to the semantics of Lisp, prints out the result, and display the prompt for the next expression to be entered. Anything that is not a list is an atom. In Lisp, numeric constants evaluate to themselves, they are examples of self-evaluating forms.
Symbols evaluate to the values to which they are bound .
Initially most symbols are unbound and thus have no value. Thus, Lisp exploits the equivalence of programs and data. Symbols and Special Forms Lisp symbols allows us to label data with names. You can use almost all the characters of the keyboard to create a symbol. Specifically, setf does not evaluate its first argument.