Purr
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Some species of felines make a sound which is called a purr. It varies in detail from cat to cat (e.g., loudness, tone, etc.), and from species to species, but can be characterized as a sort of tonal buzzing. Some cats purr so strongly that their entire body vibrates.
How felines purr
Scientists still cannot explain how cats purr. One theory holds it involves cats' false vocal cords, which do not appear to have any other function. Another theory involves blood hitting the aorta. Alternatively, purring may be caused by the vibrations of the hyoid apparatus, a series of small bones connecting the skull and the larynx that nominally serves to support the tongue. Larger cat species roar, but don't purr. Small ones purr, but don't roar.
Why felines purr
Humans usually take purring cats (particularly pet cats), to be expressing some type of feline friendliness or contentment, and most humans enjoy listening to or holding a purring cat. And, (pet) cats often, but not always, purr when being stroked by humans. One or several of these may, or may not, be the reason for making the sound; it is well-established that a cat will also purr when it is uneasy, nervous or in great pain, perhaps to comfort itself; other theories suggest that a cat purrs when it wants or needs or is getting attention, whether it be affection or medical treatment. Purring may also reduce pain, help a wounded cat to heal, and even help to keep a cat's bones strong. Cats purr at 27 - 44 hertz, and exposure to similar sound frequencies is known to improve bone density in humans. Purring has also been proven to have positive effects on cell regeneration.
Other examples of purring
It is not clear quite how and when purring is used between cats themselves, which is probably a more important issue bearing on its primary purpose than how and why it happens when humans are involved. One speculation is that it is a signalling mechanism between mother cat and nursing kittens. Female cats are known to purr while giving birth, and this may be to reduce the pain and also assist post-natal healing. Mother cats may use purring to call their kittens to nurse. Unable to see, hear or smell very well the kittens can feel the vibrations of their mother's purring.
Some cats seem to be able to meow without interrupting the purring noise.