According to the National Institute of Deafness and Other Communication Disorders (NIDCD):

A cochlear implant is a small, complex electronic device that can help to provide a sense of sound to a person who is profoundly deaf or severely hard-of-hearing. The implant consists of an external portion that sits behind the ear and a second portion that is surgically placed under the skin.

 

Illustration of cochlear implant.

 

An implant has the following parts:
 

  • A microphone, which detects sound from the environment

 

  • A speech processor, which selects and arranges sounds picked up by the microphone.

 

  • A transmitter and receiver/stimulator, which receives signals from the speech processor and convert them into electric impulses.

 

  • An electrode array, which is a group of electrodes that deliver the signals from the stimulator and sends them to different, frequency-specific regions along the auditory nerve.

 

  • A cochlear implant does not restore normal hearing. Instead, it can give a deaf person a useful representation of sounds in the environment and help him or her to understand speech.

 

http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx