It has the same construction as a normal diode plus a window and lens which focusses light upon the P-N junction.

It is operated in the reverse bias mode. 

Any change in light falling upon the junction produces a current change directly proportional to the changing light level.

Under dark conditions the current is typically 2 nA and under bright conditions 100 uA.

It has a very fast response to changes in light level.

It is frequently used in a Wheatstone configuration where a shielded second photodiode is used for temperature compensation.