An optocoupler is designed to send an electrical signal between two nodes without an actual direct electrical connection via a conductive conduit.  The most important purpose for this type of device is for electrical protection.  Since a signal is being transferred without a physical medium, the optocoupler creates an isolation in the circuit which is immune to very high voltage surges well into the thousands of volts.  A common example of this would be a lightning strike or inductive spikes.

The device itself relies on a photo transmitter and a photoreceiver or sensor.  In most of these devices, the EM transmission signal is an infrared frequency.  This is designed to take an electrical input signal, convert it to light, and then transmit.  At the other end, the sensing device detects the infrared signal and transfers to the appropriate electrical output.

Most optocouplers are designed using photoresistors or photodiodes.  Both of these devices respond to a light source and alter the current flow through their medium.  They are passive elements in this capacity so rely on a power source but when they change resistivity because of light that changing current flow can be detected and converted to an electric signal.