Tinnitus is, very simply, a ringing in the ears when there is no actual sound present.
The part of the ear that is involved with sound is known as the cochlea and is a very complicated organ. Sensory hairs, internal fluid and nerve receptors/nerves can easily be damaged and create a wide range of symptoms. As cochlea becomes damaged improper input to the brain is received which creates interference that generates a wide range of sound feedback that is inappropriate creating abnormal sounds known as tinnitus.