What is the sensor:- 

According to the instrument society of America, “a sensor is a device that provides usable output in response to a specific quantity which is the measure”. We live in the analog world. There are many sensors in everywhere, including human bodies. The sensor helps to convert data or information from one domain to another domain.

A sensor is a type of measurement used to measure environmental factors. Sensors produce the signal relating to the quantity that is being measured. And also sensor converts a physical event to an electrical signal.

What is the transducer:- 

A transducer is an advice that covert energy into one form to another form.

What is an actuator –

the actuator is the opposite of the sensor. An electronic signal converts to a physical quantity.

 E.g., using a microphone, we can convert voice to an electrical signal. Using a speaker, we can convert that electrical signal into a voice.