What Purring Means
Purring is not a part of every cat's repertoire of social communication, but it is one of the most common. Not a great deal is known about the mechanics of purring, but purring is generally associated with contentment and happiness in cats.
Interestingly, though, purring is sometimes heard in cats that are severely ill or anxious, perhaps as a self-comforting vocalization.
Vocal cats use vowel sounds to indicate their desires. The subtle differences in sound communicate commands as well as requests and complaints.
In the wild, vowel sounds are restricted to kittens, but the process of domestication has extended this method of communication well into adulthood.
Adult cats also create high-intensity sounds by changing the shape of their mouth. Hisses and grumbling are the most common examples, used primarily between cats as a means of communicating aggressive or defensive intentions. Cats in heat and feral strays also use this form of communication.