Variables
For local variables, use let:
(let ((var1 expr1)
     ....
     (varn exprn))
     body)

> (let ((x 1) 
       (y 2))
    (+ x y))
3

A let expression has two parts: a list of instructions of form (var expr) and a body - a set of expressions evaluated in order.


For global variables use defparameter or defvar:
> (defparameter *g* 9)
g
Suggestion: To avoid confusion between local and global variables, the names of global variables should be surrounded by *


Defining constants: defconstant:
> (defconstant l (+ *g* 1))


Asigning values to variables:
setf: 
> (let ((x 1))
    (setf x 2)
    x)
2
> (setf *g* 10)
10

When the first argument is a symbol that is not a variable name, it is considered a global variable:
> (setf x (list 'a 'b))
(A B)

The first argument can be an expression:
> (setf (car x) 'n) 
N
> x
(N B)

Asignation in the body of a function:
(defun foo (x) (setf x 10)) 
will not affect the variable outside the function (call by value, not by reference).


Incrementing/decrementing values of variables:
(incf x) <=> (setf x (+ x 1))
(decf x)
(incf x 10)

Other useful macros:
rotatef: interchanges the values of two variables: 
(rotatef x y) <=> (let ((tmp x)) (setf x y y tmp) nil)

shiftf: 
(shiftf x y 10) <=> (let ((tmp x)) (setf x y y 10) tmp)