The standard error output stream (stderr) is typically used by a program to output error messages or diagnostics.
The standard output stream (stdout) is the stream where a program writes its output data.
"Stderr is better than stdout for errors for a couple of reasons"
from question "Command line utility: how to make it right?"
"In other words instead of mis-using stderr like this - look into ways to gain more control over stdout"
from question "Difference in Processing of System.err.print ans System.out.print"