This is a sound that is made by all species of felids (cats). Even though, we all know that purring is a sign that a cat is content, it goes far more than that. Purring is a means of communication for cats and also means of healing.
A cat also purrs when it scared or threatened. People smile when they are nervous, when they want something, and when they are happy. So also a purr is an appeasing gesture on the part of cats.