How to Write Reference Letters

I recently wrote a reference letter for someone I supervise. When writing one of these, it can be somewhat difficult to know what to say in the letter. You want to write a good letter (unless you don't like the person who the letter is for, in which case they were stupid to ask you) so the person gets what they are applying for, but you don't want to go overboard and sound ridiculous. So, here are some commonly used terms for reference letters, and what they really mean. This can also be used as a resource to translate a reference letter so you can know what your supervisor has written about you.