Tinnitus is the phantom perception of sound. This means that a person with tinnitus hears a sound (usually a ringing or a whistling sound) that cannot be heard by others. In a very small percentage of cases, a sound actually exists (usually a vascular anomaly, like an aneurysm, that causes blood to make a swishing or squirting sound) - this subgroup of people with tinnitus does not represent the majority of people with tinnitus.
In most cases there is no source for the sound that is perceived by the patient. The truth is that we don't know, exactly, what causes tinnitus.
What we do know, however, is that tinnitus can be caused by injury to the cochlea (the organ in the inner ear that houses the hair cells that detect vibrations in fluid that are transduced by your eardrum). Tinnitus can also be caused by injury to the auditory nerve, and by injuries (strokes, for example) to the brainstem, midbrain, and cortex.
We also know that tinnitus can be caused by exposure to noise and by certain drugs that damage the hair cells in the cochlea. We also know that even when tinnitus is caused by injury to the cochlea (such as by trauma or by drugs that kill hair cells), you can remove the cochlea or cut the auditory nerve (that connects the cochlea to the brain) and the perception of tinnitus (in most people) still remains.
This fact tells us that, in most people with tinnitus, the generator(s) of tinnitus lie somewhere in the brain - even when the initial injury to the auditory system occurred in the inner ear (i.e. to hair cells in the cochlea).
Most people who study tinnitus believe that neural plasticity is involved in tinnitus. Neural plasticity is the ability of the brain to rewire itself in response to environmental stimuli and to injury.
This ability is very useful - it enables, for example, the brain to recover some or all of its function after stoke.
We think that some neurological disorders are caused when injury or environmental stimuli (like noise exposure) result in the activation of neural plasticity, resulting in one or more circuits in the auditory system to become altered - and this altered function results in the perception of sound when no sound exists.
For example, we have shown that chronic exposure to noise results in a change in the balance between excitatory and inhibitory neurotransmitters in certain parts of the auditory cortex and midbrain. This change in the balance of inhibition and excitation results in a net increase in excitation - with both theoretical and experimental evidence that this change can result in the phantom perception of sound.
The cause(s) of tinnitus in most patients is unknown, but strong experimental and clinical evidence points to neural plasticity in the brain being the reason for the phantom perception of sound.
Backstory? I have had tinnitus for close to 15 years.