A photodiode is a semiconductor device that converts light into an electrical current. It was developed from research into PN junction diodes in the 1940s and 1950s. The photodiode uses the photoelectric effect to generate a current when exposed to light. There are different types including PN, PIN, avalanche and Schottky photodiodes. Photodiodes are used in applications like optical communications, smoke detectors, camera light meters, and medical devices due to their ability to accurately detect light intensity.