Simplified Common Lisp reference
if
Symbol class: Functions, Evaluation, Flow Control, Definitions and Syntax
Syntax:
Symbol type: special syntax
iftestthenelse(optional) => an object
Argument description:
test an expression
then an expression
else an expression, default NIL

IF special form is used for branching. Either "then" or "else" branch is taken. Then branch is selected when "test" result is not NIL. See also COND, CASE. 

(if (> 3 4) "variant 1" "variant 2") => "variant 2"
(if (> 4 3) "variant 1" "variant 2") => "variant 1"
Function indexFull documentation for if (HyperSpec)