Inductive or Capacitive Proximity Sensor
So what's the difference between an Inductive and Capacitive sensor, well to start with an Inductive sensor normally have a blue cap while the Capacitive is orange. Further search on the internet reveals that a inductive sensor uses a magnetic field to detect objects which will restrict it to primarily metal, in comparison a capacitive sensor will detect dielectric materials (poor conductive materials) such as glass, plastic, porcelain, and maybe aluminum. Both these sensors come as NO (normal open) or NC (normal closed), they further come as type NPN or PNP, and these sensors normally have three wires: brown=positive, blue=ground (negative), black=signal (voltage will drop to zero when the sensor detects an object). For a better explanation of NPN and PNP please refer to the following link http://www.schneider-electric.co.uk/en/faqs/FA142566/. Basically these sensors are just an ON/OFF switch without touching a surface with proximity distance of 4 mm to 8 mm and above. You can achieve the same thing with an end-stop switch the same ones that are being used in 3D Printers, except an end-stop switch will need to make physical contact where as an inductive or capacitive sensor does not.