This is a very small light sensor . It is called light sensor or photocell or LDR (light dependent sensor) or photo-resistor. A photocell changes  resistance depending on the amount of light it is exposed to. These little sensors make great ambient light triggers (when light in the room turns on, do something). 

Features

• Light resistance : ~1k Ohm
• Dark resistance : ~10k Ohm
• Max voltage : 150V
• Max power: 100mW

Dimensions

• 2 x 4 x 5mm
• 4mm between pins
• 31mm lead length

What is LUX and what LUX value for diffrent light conditions, Click here

For LDR (light dependent resistor) datasheet and tutorial, Click here

For  GL5528 photo-resistor datasheet, Click here