Tails
Tails are probably one of the best communication indicators that cats have. One can almost read every cat feeling and thought by looking at his/her tail.

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Tail arched and puffed out or bristled - your cat is scared or fearful and is ready to attack or defend itself. The bristling is instinctual as it makes the cat look larger.
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Tail straight up in the air and fully puffed out or bristled - your cat is angered, aggressive and ready to attack.
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Tail straight up in the air and quivering a bit - your cat's tail moves this way when spraying urine to mark territory; however, once your cat is spayed or neutered he/she will continue to move his/her tail this way (without any spray or urine being excreted) when you greet him/her. This is your cats way of saying, 'hi, it's your loving cat!'
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Tail straight up and unmoving - an unabashed greeting. This is what a mother cat does to her kitten to tell him/her to inspect her rear.
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Tail straight up but the tip tilted to one side - your cat is very interested and intrigued by something; your cat is happy and friendly.
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Tail straight up or just slightly raised and curved like a question mark - your cat is very excited and interested in something.
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Tail curved down and then back up again near the tip - your cat is very relaxed and content.
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Tail still but tip of tail quivering or twitching - your cat is mildly irritated. Your cat may attack if the twitching becomes violent enough.
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Tail swishing from side to side rapidly or violently - your cat is about to attack something, this is seen during fights and play time. This is not the same as tail wagging in dogs.
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Tail held to one side - your cat is giving a sexual invitation (generally only females in heat do this).
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Tail held low and puffed out or bristled - your cat is intensely afraid.
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Tail held low and tucked between the hind legs - your cat is showing his submission or defeat.