Cats use their tails to communicate their emotions, both to other cats and to humans. Below are some typical "tail postures" you will see, and what they mean.

	If the tail is curved gently downward, then curved up again at the tip: The cat is relaxed and comfortable.
	If the the tail is slightly raised and softly curved:; The cat is beginning to get interested in something.
	If the tail is erect, but tip is tilted over, either forward or back: The cat is very interested and feeling friendly.
	If the tail is fully erect and tip is vertical: The cat is offering a friendly, cheerful greeting.
	If the tail is erect with whole length or tip quivering gently: The cat is showing affection.
	If the tail is still, but the tip is twitching occasionally: The cat is slightly irritated or pensive.
	If the tail is still, but tip is twitching intensely: The cat is very annoyed.
	If the tail is swishing vigorously from side to side: The cat is angry.*
	If the tail is straight up and fully bristled: The cat is showing aggression.
	If the tail is arched and bristled: The cat may attack if further provoked.
	If the tail is lowered and fluffed out: The cat is afraid.
	If the tail is raised and fluffed out (the "Big Tail"): The cat is probably happily chasing around.
	If the tail is fully lowered, perhaps tucked between hind legs: The cat is showing defeat or submissiveness (toward another cat).
	If the tail is held to one side, and the cat is crouched or with rump in air: This female cat is ready to mate.

Reference: http://www.xmission.com/~emailbox/tailtalk.htm