Ambient Light Sensor - The ambient light sensor will report to the handset the amount of light that is near the sensor. The ambient light sensor is utilized for automatic backlight brightness.
Proximity Sensor - The proximity sensor allows the handset to perform certain functions based on objects detected near. This allows the handset to disable the touch screen while on a call to prevent unintentional activation of on screen options during a call.
One is an ambient light sensor for adjusting the backligting on the screen automatically to the brightness of the environment and the other is another sensor that shuts the screen off when Your on the phone