What is the difference between utf8mb4 and utf8 charsets in MySQL?

What is the difference between `utf8mb4` and `utf8` charsets in *MySQL*? I already know about *ASCII*, *UTF-8*, *UTF-16* and *UTF-32* encodings; but I'm curious to know whats the difference of `utf8mb4` group of encodings with other encoding types defined in *MySQL Server*. **Are there any special benefits/proposes of using `utf8mb4` rather than `utf8`?**