And for the science bit....
Inside your ear you have lots (can't remember the exact figure) of very fine hairs. These hairs are very important as the speed and amplitude of their vibrations are basically what enables our brain to understand what it is we are hearing. To hear sounds accurately these hairs need to vibrate with the sound wave as it enters the ear, if the sound pressure level is too high (for example if the bass is too loud) the hair is vibrates past it's optimum level and the hairs get stuck in the wrong position. The ringing sound will continue until the hairs return to their "normal" position.
If you hear ringing then you have done permanent damage to your hearing. There's a formula for knowing the extent of the damage, but in general if the ringing lasts for considerably longer than the exposure time to the noise the damage is large. Say, if you were in a club for 4 hours, and in 2/3 days time you still hear ringing, then that's considerable damage, as the exposure time (4 hours) is much less than the ringing time. The ringing may stop, but you will not be able to hear as well. I think the damage is cumulative as well, i.e. each time you hear ringing you'll have damaged it more. E.g. If the hairs have bent too far at a sound pressure level of X the first time, the next time a sound pressure level of less than X will be too much for them to cope and they will get stuck in the wrong position again. The effect that this will have on your hearing is to reduce the frequency range that you can hear successfully, if you ever want to become a music producer or a musician i BEG you to look after your ears.
There's a simple solution that doesn't involve buying ear plugs. It sounds a bit silly, but bear with me... put a small piece of toilet tissue in your ear. The tissue should be enough to reduce the pressure level of the music such that the hairs aren't bent too far. If you are working in a club i'd wear ear plugs, but if you are just on a night out tissue should be fine (as long as you're not doing it every night!).
I haven't explained it very well, but i'm tired, and i did my acoustics modules a long time ago.