Lets talk about sensors bit.

Ambient Light Sensor : These sensors are used to increase/decrease brightness as per surroundings and help in saving battery life.

Proximity Sensor : These sensors are present in iphone only and switch off the display & button functions as you bring device closer to your face/ear, so that you would not press or invoke any of iPhone function accidentally during calling.

Accelerometer Or Motion Sensor : These sensors are used to measure linear movement and help in automatically adjusting screens in landscape/portrait mode based on how you are holding the phone (up,down or sideway)

Gyroscope Sensor : iPhone 4 and iPad2 is coming with another sensor: 3 axis-Gyroscope, which combines with accelerometer and provides better response on device’s movement and orientation.. particularly in gaming. Gyroscope basically measure angular motion/ rotation.

Basic understanding on Accelerometer and Gyroscope :

• You tube on sensors: 3mins
• Gyro vs Accelero