Basically it's constantly hearing this really really really really 'loud' ringing noise in one or both of your ears forever. However, even with tinnitus you can still hear very faint noises, as if the sound from tinnitus does not register in the same way. Compare the sound in videogames that you get after getting hit by a grenade or explosion to tinnitus. Except it keeps going.
I've had it for so long I don't really notice it until it's really quiet. For a while I thought it was normal and that it was just how the brain registered silence.
For me, actually hearing it is like playing 'the game'. You don't notice it until you think about it. At least for me.
Does anyone else have tinnitus?