Photoelectric sensors consist of an emitter and receiver, the emitter sends out a beam of light for the receiver to sense.  With the diffused and retro reflective type of sensors both the emitter and receiver are placed in the same housing and are configured for light to be reflected back to the sensor.

Retro reflective sensors use a target to reflect light back to the sensor and will sense an object that breaks the beam’s path.  The diffuse reflective type senses an object when the light beam is reflected back to the sensor.  Additional features as well as different form factors will vary from series to series.