This article is from the Tinnitus FAQ, by firstname.lastname@example.org (Mark Bixby) with numerous contributions by others.
Tinnitus can be described as "ringing" ears and other head noises that are
perceived in the absence of any external noise source. It is estimated that
1 out of every 5 people experience some degree of tinnitus.
Tinnitus is classified into two forms: objective and subjective. Objective
tinnitus, the rarer form, consists of head noises audible to other people
in addition to the sufferer. The noises are usually caused by vascular
anomalies, repetitive muscle contractions, or inner ear structural defects.
Subjective tinnitus is much less understood, with the causes being many and
open to debate. Anything from the ear canal to the brain may be involved.
Hearing loss, hyperacusis, recruitment, and balance problems may or may not
be present in conjunction with tinnitus.