What exactly is tinnitus? What is it's cause, and why do some people hear it and others don't?
Although tinnitus is usually described as a ringing in the ear, there's a whole range of tunes, buzzes, whooshing sounds, humming and hissing sounds that are described of as tinnitus. The sounds can either genuinely be there or be perceived to be there. If it is genuinely there it suggests muscles ate at play or some blood vessel disease if the sounds are in time with your pulse, but usually its the latter perceived to be there that presents.
Tinnitus that is perceived and permanent usually suggests damage to your hearing nerve (called the auditory nerve or cranial nerve 8 or vestibulocochlear nerve). The common cause for this we see is patients that have been exposed to loud noises or drugs like certain strong antibiotics which cause damage to the nerve. Other drugs also have a side effect of causing tinnitus. However any ear problems really may cause tinnitus including ear infections, too much wax, getting older, a disease called Mennieres, tumours, neurological diseases, vitamin deficiencies and lots more.
If you're really interested, keep reading. There's a number of reasons why it might occur. Usually patients presenting with it have some degree of hearing loss. Its thought that somewhere along the line the structures are getting more sensitive to sound because through a feedback mechanism. Think of it this way, you speak to someone but they can't hear you so you speak louder and louder until they can. In the same way its thought that the sensitivity is increased and increased to try to restore hearing but the result is audible nonsense sound from abnormal activity i.e. tinnitus.
If anything doesn't make sense, please do say :)
+1 for giving a solid answer to @AP, but being older I've had friends who have had tinnitus and I'd like to add some notes to try to flesh this out a bit.
I don't think tinnitus is the result of nerve damage usually. Nearly everybody experiences episodic tinnitus at one point or another. When exposed to a loud sound or a blow to the head can cause it.
Chronic tinnitus occurs when the sound does not go away and is heard all the time. This is annoying but incredibly common - nearly 1 to 2 in ten people have an extended period and about 0.5% of people experience tinnitus to the point where it affects their daily life and it doesn't seem to ever go away.
Like headaches, schitzophrenia, depression, and other neurological conditions, the specific cause of tinnitus is not really understood and explanations which are really satisfying are hard to imagine.
While a loud noise could possibly cause nerve damage in the auditory system, it sounds more often the cause is related to a vulnerability in the auditory system that allows the generation of these sounds in such a way that they cannot be tuned out. There have been genetic factors associated with the tendency to tinnitus, implying that physiological factors are in play. Tinnitis can be aggravated by stress, caffeine and smoking, which further implies that nerve physiology has a lot to do with the condition.
Much of the treatment for tinnitus involves amplifying other sounds in other parts of the sound spectrum so that the ringing sound tunes itself out - that the nervous system actually attenuates the sound as it should so that the patient does not consciously register it. Hypnosis and cognitive behavioral therapy have also been touted as well as herbal remedies, diet changes, but attenuation is all the result that is typically expected.
There is a class of tinnitus where a pulsating sound from vasculature near the auditory nerves appears to be a cause.
If you want a great talk that generally covers the molecular mechanisms behind hearing, I highly recommend this one by James Hudspeth. This covers a ton of material, including otoacoustic emissions, and is very interesting.