sensor and transducer

A sensor produces an electrical output in proportion to the particular thermal/mechanical/pressure input. Think thermocouple.

Transducers convert electrical energy to mechanical energy in proportion to input. Think loudspeaker.

Piezo electric materials go both ways. Mash it, it produces current. Input current, get motion.

I'd Wiki if I were you.

The terms are used interchangeably.

sensor: senses the change in the parameter to be measured.It's output need not to be a standard one (i.e 3-15 psi or 4-20 mA etc) Transducer:A sensor with proper signal conditioning to produce output in a desired standard range constitutes a Transducer.