The command frame is used to interpret the user inputs.

The user inputs there answer using the front touch screen.

The program loads itself into the background and monitors user input.

This user input increases the variability because of possible human error.

I think it's really important to get user input early on, even if you are working in an Agile way.

This program will take a user input, and count down to zero.

This program will take a user input, square it, output the answer and then repeat.

But very complex web pages that change in response to user input require programs to make them work.

Invalid - the system should respond to user input within 10 seconds.

Most traditional software programs respond to user input and do nothing without it.