Body language is a cat's main form of communication with other species. Therefore, it's important to recognize certain signals in order to understand the animal's moods and feelings and react accordingly.
When a cat feels content it points its ears forward and turns them slightly to the side. In this case, the whiskers are spread out and the animal makes an overall relaxed impression. That doesn't mean, however, that the cat's owner can pet it at will. When a cat wants affection it will usually indicate this by approaching a person with its tail pointing upward. If the cat curls up in your lap, it is positively desperate for some love!
Attentiveness in a cat is apparent when its ears and whiskers are stretched forward and its whole body appears tense. The cat is in a defensive pose when its ears are folded to one side, its pupils are enlarged and the teeth are slightly visible. Its fur stands up and it might start to growl, snarl or hiss. If the cat is preparing to attack, it will stand tall and stretch its legs, open its mouth wide and narrow its pupils to a slit.
With a little experience it's not hard to recognize the signals of a cat. There are many expressions and gestures that can greatly facilitate the relationship between a cat and its owner, provided the latter is aware of them.