What are "$\mu$-recursive functions" and $\mu$-calculus? I saw in this question a reference to $\mu$-recursive functions or $\mu$-calculus as some computation model equivalent to Turing machines and $\lambda$-calculus. I know about these two but never heard of $\mu$-calculus. Can you briefly describe it and compare it with the other models? Bonus points for link references and books for beginners. Thanks in advance.

Edit: Ok, just for clarification i'm interested in both $\mu$-recursive functions and $\mu$-calculus.
If it is the μ-recursive functions you are interested in (and not the μ-calculus, which is a totally different thing as slimton pointed out) it makes the most sense if you start looking at the primitive recursive functions (PRFs). The set of PRFs is interesting in it self and it turns out that the μ-recursive functions can be introduced by an extension of it.

Here are some slides that seems to be a relatively gentle introduction:
Link