This is a very small light sensor.
A photocell changes (also called a photodetector, photo resistor, CdS or photoconductive cell) 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

Documents:

• Datasheet
• Bildr Tutorial