Ntstanch,
You've probably read this, but tinnitus is where the little hairs in your ears that process sound are damaged via some mean (a really loud explosion, exposed to constant loudish noise, ear infections, hitting your head, and etcetera).
I was only "diagnosed" with such as I damaged my ears by firing a pistol indoors without hearing protection, and my ears were very sore, so I went to the doctor's place to see how they were; the ringing I experienced was said to be such. It's been there ever since. It doesn't bother me, as I've gotten used to it.