In the short form we call it LDR, means light dependent sensor. Photo resistors are recognized as a light sensitive component used in project where we have to measure the intensity of the light or we need to find if there is light or absence of the light. Means using this sensor we can detect the presence of the light and its intensity. On the other hand we can also detect whether there is dark. A single device can be used to measure the presence of the light. While using, In the darkness resistance becomes very high and when it is exposed to the light then its resistance starts dropping instantly and results can be seen.
They are very cost effective and easy to interface with any microcontroller or circuitry. But now days Photodiodes and Phototranistors are used as a substitute of LDR or photoresistors. Few countries have banned using the photoresistors due to the material cadmium used to manufacture this because of the environment and health hazards. But even if it is used all overworld when we have to work with light related project or for any industrial microcontroller based development.