Nike + iPod is a built-in app in the iPhone, iPod touch and iPod nano that allows you to pair a Nike+ Sensor to track your workouts. The Nike+ Sensor is sold separately.
You can enable Nike + iPod in the settings of your device. Look for the Nike + iPod icon on your device display.
Nike + iPod requires a Nike+ Sensor to measure the distance of your run. If you want to run without the sensor, or you want to utilize the GPS functionality on the iPhone, you can download the Nike Run Club app.