User Interface, http://en.wikipedia.org/wiki/User_interface Last accessed 28/05/06

User interaction is basically were a user the person using the device interacts with a machine or object. With games the user is not only interacting with the games system but with objects within the game as well.
Because the user is interacting on so many levels it best to try and keep things as simple as possible so not to confuse the user while limiting the amount of effort they need to put in, since if the game is too taxing and requires a lot of effort the users cane be put off quickly.