This is a Sharp Distance Measuring Sensor unit 10 to 80 cm – GP2Y0A21YK0F.

This SHARP distance sensor bounces IR off objects to determine how far away they are. It returns an analog voltage that can be used to determine how close the nearest object is. Comes with 12″ long 3-JST interface wire. These sensors are good for short-range detection. For over 1 m distance, we suggest using sonar sensors

To use, connect the black wire to ground, red wire to 5V and white wire to the analog input. The analog voltage out will range from 3V when an object is only 4″ (10 cm) away and 0.4V when the object is 32″ (80 cm) away.

Note: 

This sensor draws current in large, short bursts, and the manufacturer recommends putting a 10 µF capacitor or larger across power and ground close to the sensor to stabilize the power supply line

Package Includes :

1 x Sharp Distance Measuring Sensor unit 10 to 80 cm.

1 x Connecting cable.

Useful Links:

Arduino library for distance sensors