The photodiode is, in essence, the reverse of the LED. In fact, depending on their design, LEDs can be used as a type of photodiode. Photodiodes are responsive to light in one of two ways. The first method is the photovoltaic mode. In this mode, a voltage appears across the PN junction that is proportional to the amount of light striking it. It can be thought of as a small voltage source or battery. The second mode is photoconductive. In this mode, the photodiode is reverse biased by an external DC supply. The amount of current flowing through the diode will be proportional to the amount of light striking the junction. Typically, this current will pass through a series resistor to create a voltage or it can be sent into a current amplifier circuit.
A photo emitter/detector pair is a pairing of an LED and a photodiode that are designed to produce and detect the same wavelength of light. The wavelength of light may be outside the range of the human visible spectrum. Infrared (IR) is often used for consumer remote control devices. Emitter/detector pairs might use a phototransistor in place of a photodiode. The performance is similar except that photodiodes tend to have a quicker response while phototransistors tend to produce higher currents.