let and let* create new variable bindings and execute a series of forms that use these bindings. 
let performs the bindings in parallel and let* does them sequentially.

The form

 (let ((var1 init-form-1)
       (var2 init-form-2)
       ...
       (varm init-form-m))
   declaration1
   declaration2
   ...
   declarationp
   form1
   form2
   ...
   formn)

first evaluates the expressions init-form-1, init-form-2, and so on, in that order, saving the resulting values. 
Then all of the variables varj are bound to the corresponding values; 
each binding is lexical unless there is a special declaration to the contrary. 
The expressions formk are then evaluated in order; the values of all but the last are discarded 
   (that is, the body of a let is an implicit progn).
let* is similar to let, but the bindings of variables are performed sequentially rather than in parallel. 
The expression for the init-form of a var can refer to vars previously bound in the let*.

The form

 (let* ((var1 init-form-1)
        (var2 init-form-2)
        ...
        (varm init-form-m))
   declaration1
   declaration2
   ...
   declarationp
   form1
   form2
   ...
   formn)
first evaluates the expression init-form-1, then binds the variable var1 to that value;
 then it evaluates init-form-2 and binds var2, and so on. 
The expressions formj are then evaluated in order; 
the values of all but the last are discarded (that is, the body of let* is an implicit progn).

For both let and let*, if there is not an init-form associated with a var, var is initialized to nil.

The special form let has the property that the scope of the name binding does not include any initial value form. 
For let*, a variable's scope also includes the remaining initial value forms for subsequent variable bindings.


Examples:

 (setq a 'top) =>  TOP
 (defun dummy-function () a) =>  DUMMY-FUNCTION
 (let ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (dummy-function))) =>  "INSIDE TOP TOP" 
 (let* ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (dummy-function))) =>  "INSIDE INSIDE TOP" 
 (let ((a 'inside) (b a))
    (declare (special a))
    (format nil "~S ~S ~S" a b (dummy-function))) =>  "INSIDE TOP INSIDE"