Nearly all cats purr. It used to be mostly a mystery as to how cats purr and why. New research has some clues to the purpose of purrs.
Cats generate a rumbling vibration that seems to come from deep in the throat or near the heart.
The exact mechanism that creates the sound vibration is still not known, but most vets would tell you that it may be the vibration of blood vessels rather than something vibrating in the cat's larynx. Tigers and a few rare species do not purr, as far as we can tell. It seems that domesticated cats are by far the most common purr-ers. That very question was the drive behind the research about purring. This research reinforced previous studies confirming that exposure to frequencies of 20-50Hz strengthens human bones and helps them to grow.