A cat's purr: What it means
Your cat is snuggling up next to you as you watch TV. As you gently pet his soft fur, you feel the vibration of his purring. To the average person, purring is a sign of a happy cat. However, your cat may purr in a number of different situations and may use purring to communicate.
When your cat may purr:
Behaviorists believe that the original purpose of purring was for a mother and kitten to communicate with each other. Kittens are born blind and deaf, but a mother cat can communicate with her young through the vibrations of her purrs. Because they cannot meow while nursing, a kitten might purr to tell his mother "all is well." Purring also works as a bonding mechanism between a kitten and its mother, as the mother may purr back in reassurance.
Purring behavior learned as a kitten carries into adulthood and this may be why your cat purrs while being petted — he's letting you know "all is well."
Adult cats purr when approaching other cats to let them know they are friendly.
Cats purr when they are afraid or distressed, possibly as a way of reassuring themselves.
A cat's smile: Many people have likened a cat's purr to a human's smile which is a good analogy. People smile for many emotions: happiness, nervousness, fear. Just like a smile, purring may indicate various emotions and doesn't necessarily always indicate happiness.
Fun fact: Many cats stop purring at the sound of running water. Your veterinarian may use this trick in the exam room in order to get a better sound of your cat's heartbeat which can be difficult to hear while your cat is purring.
Drs. Foster & Smith Educational Staff. "Why Do Cats Purr?" Why Do Cats Purr? 3 Reasons Kittens & Cats Purr. N.p., n.d. Web. 15 Jul. 2013. "Why And How Do Cats Purr?" Everyday Mysteries: Fun Science Facts from The Library of Congress, n.d. Web. 2 Aug. 2013.