title: What does `nix-darwin` provide?
body: <p>I am new to nix world, and I use Mac OS X. I've just installed nix on my MacBook and want to use nix to manage some packages like Emacs instead of Homebrew.</p>
  
  <p>I've found this repo: <a href="https://github.com/LnL7/nix-darwin" rel="noreferrer">https://github.com/LnL7/nix-darwin</a> , and I am curious:</p>
  
  <p>What this repo provides for a Mac user? </p>
  
  <p>Does it provide a way to manage OS X system itself? Or does it provide a way to manage softwares on OS X system? </p>
  
  <p>What does the <code>darwin-rebuild switch</code> do to my system? Is that revertable?</p>