Phototransistors are a form of bipolar transistor that is sensitive to light - having the gain of a transistor they are much more sensitive than photodiodes and find uses in many light detection circuit designs.

Optocouplers (also known as optoisolators, photocouplers, or optical isolators) are electronic components that use light to transfer electrical signals between two separate circuits. 
Galvanic isolation is a design technique that separates electrical circuits to eliminate stray currents.
The TLP521is an optocoupler that consists of a photo-transistor is optically connected to a gallium arsenide infrared emitter diode. When the IR LED is turned on, the light from the LED is collected by the transistor's receiver and it conducts. This device is suitable for applications requiring galvanic isolation, such as Switch Mode Power Supplies, relay driver boards, etc.

 

SPECIFICATIONS