Cat Communication through Tail, Ear and Eye Signals
Apart from being able to make over 100 vocal sounds with which to communicate, which incidentally is ten times more than dogs, cats are also able to communicate extensively through tail, ear and eye signals, and cat owners can get a good idea of their cat’s mood/emotions by understanding these various signals.
Tail Signals: 
Contented Signs
Tail curves gently down and then up again at the tip: This is a relaxed cat at peace with the world.
Tail raised slightly and softly curved: Cat is getting interested in something.
Tail held erect but with the tip tilted over: Cat is very interested and is in a friendly, greeting mood, with slight reservations
Tail fully erect with the tip stiffly vertical: Intense greeting with no reservations. For adult cats this is borrowed from the action of a kitten greeting its mother. The kitten's signal is an invitation to the mother cat to inspect its rear end.
Tail held erect with its whole length quivered: Gentle quivering action often seen after a cat has been greeted by its owner. The same action is observed when urine spraying is taking place outdoors, but in this case no urine is produced. The gesture appears to have meaning of a "personal identification" as if saying "Yes, this is me!"
Aggressive Signs
Tail swished violently from side to side: This conflict signal of tail wagging is its most angry version. If the tail is swinging vigorously from side to side, it usually means that the cat is about to attack.
Tail held still, but with tip twitching: This version of tail wagging shows only mild irritation. If the tip twitches become more powerful, then it is a clue that a swipe from a bad tempered paw is imminent.
Tail held straight and fully bristled: Signal of an aggressive cat.
Unhappy Signs
Tail lowered fully and possibly tucked between hind legs: Signal of defeat to totally submissive cat showing its lowly social status.
Tail lowered and fluffed out: Cat is indicating active fear.
Tail arched and bristled: Signal of a defensive cat and one that may attack if provoked further. Bristling fur makes the cat look bigger and may deter the enemy if the defensive cat is lucky.
Sexual Sign
Tail held to one side: Sexual invitation of a female cat in heat. When she is ready to be mounted by the male, she conspicuously moves her tail to one side. When the tomcat sees this he knows he can mount without being attacked.
Ear Signals:
Relaxed
Cats will usually sit with their ears facing forward and slightly tilted back as they listen for interesting sounds. When something interesting is detected, the ears change to alert mode.
Alert
The ears will then become more ‘pricked’ as the muscles in the forehead pull them in. If the cat’s ears begin to twitch the cat is most likely to be a little anxious and unsure of the noise or situation.
Agitated
If a cat is feeling apprehension, frustration or in a state of conflict, this will generally be shown by a nervous twitching of the ears.
Defensive
A defensive cat will show this by displaying his ears fully flattened. This is also a defence mechanism so that the ears won’t get hurt if the cat gets into a fight.
Aggressive:
The most dangerous ear signal a cat can display. The ears are rotated but not fully flattened and the back of the ear is visible from the front.
Eye Signals:
The slow eye blink is one of the most wonderful of cat body signals, but one that is often missed by even the most observant of cat owners. This is a very powerful communication and a reassurance signal. It is commonly used between cats when they are sitting or lying in a hunched-up ‘sphinx-like’ position. The slow eye blink is also a marvellous way to say "I love you" to your cat.
A continuous stare has the opposite effect on cats. It is threatening and unsettling and is used by cats in maintaining territorial distance. This could explain why, when a cat enters a room of strangers, all of whom are cat lovers except for one, the cat will choose to approach the non-cat lover, the only one who is not staring at him.
In a standoff situation a defending cat's pupils will be dilated to provide the cat with a wider peripheral vision, an advantage in anticipating an attack. The aggressive cats pupils narrow to give him better depth perception, an advantage in judging where to attack.
While dilated pupils can indicate fear, they are also indicative of the light conditions. In dim light the pupils expand to allow as much light to enter the eye as possible.