As
Milen's excellent write-up stated, purring is not only designed to express contentment, but
the full gamut of emotion. Purring is also useful for practical purposes in the
feline world, too.
When a kitten is very young and and hasn't opened its eyes yet, the mother cat purrs as a homing device for her babies. The kitten can feel his/her way to the vibrations and food, and they in turn purr while drinking, to let the mother know he/she is getting the milk. It's a commonly held but erroneous idea that the kitten is purring because it is content, as it is just telling the mother that everything is cool, please continue producing the goods. Mmm, milk.
Cats purr when they're unhappy and sick as well as relaxed and happy. Generally, the only time cats don't purr is when they're asleep; if you can hear your feline friend starting their little motor when they've got their eyes closed, curled up on your lap, they're just faking sleep and are napping. Cats have been known to purr while giving birth ("Oh yeah, I just love it when I go through this intense pain. Labour is the most rewarding, content feeling ever! ) or immediately after a car accident. One of our dear departed cats, Tiger, purred as he died; at the time I thought he was happy, now I know that cats purr when they're in terrible pain. Sometimes it's best not to know.
Cats also purr to show their dominance amongst other cats. The alpha cat can purr when approaching inferior cats, or, seemingly contradictorily, a sick or lower-level cat can purr when getting near superior or healthy cats to show their inability and unwillingness to fight.
277 Secrets Your Cat Wants You to Know claims there is a tape available which "guides relaxation by combining the sounds of human and cat purring with a new age musical background". According to the distributors, purring with your cat creates a closer bond, and makes you as content and happy as your feline pal. The tape costs US$10; put it on and see how long your cat stands in front of you, watching, before he turns away in disdain. Then he'll purr because he's quite obviously the superior being, you numbskull.