Arguments

expression  (optional) any Lisp expression or value; if omitted, NIL is used
fileHandle  (optional) handle of a file as obtained from (open); if omitted or NIL is used, output goes to command line

Return

the evaluated value of expression, or a "void" value

Example

(setq x 123)
(princ x)  123, to command line
(princ x fh)  123, to opened file "fh"

Remarks

1.
if no argument is provided (prin1) a "void" symbol value is returned (effectively "nothing");
(princ) can be used as last statement in a defun scope, to return "nothing", so no result is finally output to command line

2.
other than (prin1), (princ) does not use the "\n" representation for any control character (\t \n \r \" \nnn octal) when encountered in a string expression