A cadmium sulphide photoresistor (CdS cell) changes the resistance circuit depending on the amount of shining on it. When lots of light on a CdS cell, resistance is low, so it conducts electricity well. When there is little light, the photo-resistor has a high resistance, so not much current can flow. The change in current can be used to control a relay.
A relay is an electromagnetic switch; when the electromagnet has a high current (lots of light falling on the photo-resistor) it pushes the switch open so no current can flow to the streetlight. When it gets dark, not much current can flow to the electromagnet so the switch closes and allows electricity to flow to the streetlight, turning it on.