Photo taken from http://www.keynshamhearing.co.uk
A cochlear implant (CI) is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing in both ears.
Cochlear implants bypass the normal hearing process; they have a sound processor that resides on the outside of the skin (and generally worn behind the ear) which contains microphones, electronics, battery, and a coil which transmits a signal to the implant. The implant has a coil to receive signals, electronics, and an array of electrodes which is placed into the cochlea, which stimulate the cochlear nerve.
(Taken from Wikipedia.org)