These CdS Photoconductive Cells are used to measure light. They change their amount of resistance according to the intensity of the light they see. This particular GL5528 type photocell has a good range for general purpose use. The active surface of each sensor is about 5mm in diameter and they are breadboard friendly.
These photocells can be used for a variety of purposes where sensing the amount of light is important. They are often used to create photovore robots that seek the light and photophobe robots that are afraid of the dark. I'm sure you can think of lots of cool ways to use these light sensors with your robot projects.