In electronics, an Opto isolator is also termed as an optical isolator or photocoupler or Optocoupler. It is one of electrical and electronics components used for transferring electrical signals from one circuit to another circuit (which are isolated) using light. In general, combination of LED (source) and phototransistor (sensor) is used in typical Opto-isolator IC packed in a single opaque package.
In fact, there are few other sources such as LED, lamp, etc., which are detected by sensors such as photo diode, LASCR, photoresistor, and so on. These Opto-isolator circuits are used generally for transferring digital signals, but can be used to transfer analog signals with a few techniques. The rapidly changing voltages or other electrical parameters on one side of the circuit may cause damage to components on the other side of the circuit due to direct electrical contact. So, opto isolator circuit can be used to isolate the high or rapidly changing voltages of one side of circuit causing damage to the components on the other side of the circuit.