An optoelectronic device which combines an optical transmitter and receiver in a single package; commonly used in galvanic-isolated control applications such as power supplies.

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.

PD optocopuler image from Wikipedia

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.

Further reading
Wikipedia article on opto-isolators
Vishay application notes (optoisolator manufacturer)