Actuator
Actuators are outputs that produce an action in the physical environment by transforming energy into motion. Examples of actuators include: servo motor, vibration motor, etc.
Input
Inputs gather or receive data. Sensors are inputs that gather data from the physical environment. Examples of inputs include: button, thermometer, motion sensor, light sensor, microphone, accelerometer, etc.
Output
Outputs send data or do something in the physical environment. Examples of outputs include: lights, display screen, speaker, motor, etc.
Sensor
Sensors are inputs that gather data from the physical environment by detecting an event (is a button being pressed, is motion occurring, etc.) or by measuring a property (what is the temperature, how much light is present, etc.).
Transducer
Transducers are mechanisms that transform one form of energy into another form. Physical outputs (such as lights, speakers, motors, etc.) are examples of transducers because they transform electrical energy into another form (such as light, sound, motion, etc.). Similarly, physical inputs (such as light sensor, microphone, etc.) are also examples of transducers because they transform other forms of energy (such as light, sound, etc.) into an electrical signal.