A transducer is a device that converts one form of energy into another.

Microphones contain transducers to convert the kinetic energy of air movement into electrical voltage. Your ears also contain transducers (in the form of the cochlea)--they convert kinetic energy (once again, the movement of air) into (again) an electrical signal. This one is a little different because it's meant to be sent to the brain. The process is similar, however.