Acceptance With Grace. The best_case scenario__and honestly, the most common reaction__is that your boss will accept your resignation with understanding and sincere congratulations. Your manager will be happy to see that you're advancing your career and moving on to something bigger and better.
A resignation letter can also help your boss and your company start the transition process, get a replacement for you approved, and start planning internally, etc. The paper trail is beneficial all around. So what should go into your letter? Let's look at some examples of what to do (and perhaps more importantly, what not to do).