P.S. Please correct the questions. It is not:
Difference between ?System.out.println? and
?System.error.println??

It must be:
Difference between ?System.out.println? and
?System.err.println??

The java.lang.System provides 3 static fields System.out,
System.err and System.in which represent the standard output
stream, standard error stream and standard input stream

Standard Output stream connects the program to the system
console screen and is commonly used to display informative
messages to the user.

Standard Error stream by default connects the program system
console screen and is commonly used to display error
messages to the user.

The Standard Input stream connects the program to the system
keyboard device and provides capability to the program to
read from keyboard input.

Java provides facilities by which these streams can be
redirected to other sources or destinations and change the
way the program behaves.

For example, the standard error stream can be redirected to
a output stream that writes to a log file for error analysis.

out.println will print the output to an output stream.
err.println will print the errors to the stream