If you have tinnitus, you will be hearing an internal noise all the time. If it has gone on for several months, you can be pretty sure it's tinnitus.
You might hear a whistling, a ringing, a hissing, a thumping, or other kind of sound, but the thing that distinguishes it is that it is completely inside your head, and so has nothing to do with external noise. In other words, only you can hear it.
Strangely, in a study which entailed putting people into a sound-proof box where they couldn't hear any external noises, it was discovered that they all could hear some form of internal noise.
The thing with tinnitus is that the normal filtering that goes on to stop you from hearing the internal noises has been damaged in some way, so that now you do hear it, and this can be very distressing.
Damage can occur in several ways. For example, I hit my head during a faint and gave myself tinnitus. Certain medicines can also cause it. But by far the most common cause is exposure to loud noise. Sadly, many young people are unaware of this, and end up with tinnitus from visiting a nightclub where the music was very high, or even from playing their ipods too loudly. Sometimes, depending on the amount and level of exposure, the internal noises will disappear after a few days or weeks, but for some the damage will be permanent.
Rarely, tinnitus can be one of the symptoms of certain other condtions, which is why it is a good idea to see a doctor, just to rule out anything else which may be causing it.
While there is no cure at the moment, tinnitus can sometimes be alleviated by things like drug treatment, maskers (which produce white noise), ginko biloba (a herb found in the health food stores), or by concentrating very hard on other things so that you train the brain not to listen as much to the noise. But what works for one person may not work for another, and it's trial and error as to which method will help an individual.
Working with an ENT specialist should produce the best result.