Arduino Boards With Sensors:

Not only can Arduino deal with simple programming to control its outputs, but it can also have sensors attached and allow inputs to control how the board behaves. (briefly talked about in precious post)

For example, If a infra red sensor was attached to the board, and the board was programmed so that once the infra red detects an object in its path, the board could control something it is connected to to change.

A good demonstration of this example is an Arduino board hooked up to a remote control car, only the cars steering is controlled by the board and the board changes direction depending on the infra red sensor. So, if the infra red detects an object in the cars way, the arduino board changes direction, but if all is clear the car will continue to travel until the sensor detects otherwise.