Cats are different from all other animals in the sense that cats have the unique ability to purr.
This odd sound a cat makes is akin to the vibrations of a automobile motor running on idle, While most people are aware that cats purr, they may not have a clear understanding of why a cat will purr. So, to dispel a few myths as to why cats do this, here is a brief overview of what purring is as well as its purpose.
Oddly, despite many decades (if not hundreds of years) of veterinary medicine study, it has never been understood how cats are able to make a purring sound. The biology of a cat is not far removed from many other mammals. Yet, cats have the special ability to purr while other animals are completely unable to do this. Why do cats develop the ability to purr when others do not? Most likely, it is the natural evolutionary development of a trait necessary for survival. Specific evolutionary traits are common among all animals and with cats purring is among several behavioral traits that these pets have embodied.
For example, have you ever wondered why cats like to perch atop high places? This is a survival mechanism that has allowed cats to survive through thousands of years through evading predators. By being perched high, cats can easily scan movement in a very expansive perimeter. This way, they can spot encroaching predators (and potential prey) almost immediately. Such traits have become ingrained in cats over the years as a means of self-preservation. Purring is a similar derivative of such behavior.
Why do cats still do this when they reach adulthood?
When cats where first integrated into people's homes as pets, they were not completely sure what to make of their human owners. Cats are not pack animals and prefer to be solitary. However, they do certainly appreciate the human that opts to care for and feed them. Now, cats have a very strong memory and can recall their time as a kitten. As a result, they will look at a human being as a surrogate mother. As such, when the cat's owner allows the cat to sit on his/her lap or pets the cat, the cat will start to purr. This is a sign of happiness, safety, and contentment in the presence of an owner. Once again, the cat is conveying the message that it is ok. Any why should it not be? It is in the presence of a loving owner that cares for the cat's every need!
When a cat is born it is both mute and blind. This makes its ability to communicate with its mother difficult. So, when the kitten is distressed, it will not be able to let the mother know it is sick or ill. Conversely, the kitten cannot tell its mother that it is doing well through meowing or through its behavior. As a result, cats developed the ability to purr. By purring, the kittens convey to its mother that it is fine and doing well.
So, when you hear your cat purr, you can rest assured that your cat is happy, safe, and displaying affection towards you. Consider it a cat's version of a term of endearment.