Tinnitus is, by definition, a subjective ringing or tingling in the ear that can only be heard by the person experiencing it. The noise can be described in many ways: a humming, buzzing, ringing, whistling, clicking, throbbing, or roaring. It's often a symptom of another underlying problem (see Causes, above, for a list).
Many people with tinnitus also have lost some or most of their hearing. In some cases, it's the hearing loss that ends up causing the problem.
The worst part of tinnitus is that it can be very annoying and difficult to live with. A soft ringing sound might not be too bothersome to most, but some people hear very loud sounds over long periods of time or sounds that go away, only to return just as they get used to the silence again. This can affect sleep, concentration, and quality of life.