There are three things that happen when you interact with the game - Input, Process, and Output. Input is when you press a button or move the mouse and so on. When you do this, you interact with the computer or system game. With no Input, the game will not have anything to interact with. Process is what makes the decision on what's going to happen in the game after you put in Input.
Output is the last quick step in game interactivity.
That is when the computer actually does the command, such as displaying a movie, playing a sound, or moving a game character. The cycle of interactivity keeps looping around until the game ends or the computer is shut off.