Definitions
Sensor : a device that senses one of the fundamental physical
variables.
Transducer : a device that changes one form of energy to
another.
Actuator : a device that performs an action on one of the input
variables of a process according to a signal received from the
controller. (Example Valve, Magnetic relay)
Transmitter : conditions the signal received from transducer so
that it is strong to be sent from distance away to a receiver such as
a controller.