Final answer:
Error messages generated by commands are sent to STDERR by default. STDERR is the standard error output stream and it is separate from STDOUT, which is used for the main command output.
Step-by-step explanation:
Error messages generated by commands on a computer system are sent to STDERR (standard error) by default. This is different from STDOUT (standard output), which is where the main output of a command goes. STDERR is designed to separate error messages from the main output so that they can be handled differently, for example, redirected to a file or displayed to the user. Log files are not the default destination for error messages, although they can be used to store the output from both STDOUT and STDERR if configured by the user. STDIN (standard input) is used for input to a command, not output.