Cats use their tails for more than just maintaining their balance. Many times the posture/position or movement of a cat's tail can give a clue to your cat's mood.

   If a cat swishes its tail right and left it is definitely irritated and
not happy. If the tail goes straight up in the air with no movement is the sign of a happy cat. If your cat becomes fearful or threatened the hairs on tits tail will stand up.  When cats fight with each other their tails get inverted in an L or V shape. You will also see this posture in kittens who are engaged in fierce play.