Because out is a reference variable of PrintStream class;the value returned by System.out must either be null or an object of class PrintStream. When JVM starts an application running,it instantiates an object of PrintStream class & connects it to standard output device(Command prompt) When PrintStream object is instantited by JVM,its reference variable is assigned to the "out" class variable of the System class. Thus System.out returns an object of class PrintStream & in class PrintStream there is a method called println() which prints the string on screen. So by System.out.println() ; we are able to print characters on screen.
I dont know much about err variable & whats its use.Can anyone tell?