Optoisolators are often used for control purposes, to convey information between two isolated circuits.
They are very commonly used in switching power supplies to transfer information between low-voltage secondary circuits and mains-connected primary circuits (and vice versa.)
A typical optoisolator contains a photodiode and a phototransistor in a light-proof sealed package, with a gap between the components. The gap provides galvanic isolation between the source and the receiver.
A source signal is applied to the photodiode. The source current generates a certain amount of light, which travels across the gap to the phototransistor, leading to a certain amount of current gain in the transistor. The ratio between transistor current and source current is called the current transfer ratio or CTR, and is a critical optocoupler parameter.
The term opto-isolator is synonomous with optocoupler - the terms are used interchangeably in industry.