Why do cats hiss? A hissing cat is basically trying to frighten away whatever or whoever he's hissing at.
Hissing is an act of defense. The cat is issuing a clear warning saying "carry on approaching me at your own risk."
Snakes are one of the most deadly and feared animals. A cat hissing resembles a snake that's just about to strike - both facially and in the way its body moves.
Mimicry - where one animal looks or acts like another - is common amongst different animal species. Some cat behavior experts believe that cats hissing came about through mimicry of snakes and has evolved as a feline defense mechanism.
It's easy to spot the similarities here...
So why do cats hiss and mimic snakes, you may well ask. A cat that's hissing is trying to frighten whatever it's hissing at. This usually happens either because it's scared of being attacked, so is trying to "face off" the would-be attacker, or it's showing aggressive behavior as an act of dominance.
A cat that's showing this sort of behavior is a pretty frightening sight (he's intending for it to be).
The best advice I can give if you're faced with this is:
Most cats - possibly with the exception of un-neutered tom cats - don't actually want a fight. They don't enjoy being hurt or the prospect of it.
Their reason for hissing and all the other aggressive body language that goes with it is to try and frighted off the person / cat / dog etc. who it's directed at so that a fight can be avoided.
For this reason, if you follow my advice above, you'll stand the best chance of coming away from the exchange unscathed. Click here for more detailed information on dealing with hissing cats.