Standard File Descriptors/Handles

On Linux, other Unix-like systems, and on Windows, programs may open file descriptors (Linux terminology) or file handles (Windows terminology). Each file descriptor/handle is a numbered channel connected to a file or device.

By default, three channels are opened automatically by the shell when a process is started. These are:

0 - Standard Input (stdin) - this is the default input channel for 
    the program

1 - Standard Output (stdout) - this is the default output channel 
    for the program, used to output "normal" messages

2 - Standard Error (stderr) - this is the default error channel 
    for the program, used to output error messages

Without redirection, all three of these descriptors/handles are connected to the terminal. Therefore, the command will get input from the terminal, send output messages to the terminal, and send error messages to the terminal.