A letter is a special kind of note (see "help notes") with the added feature that it can be recycled by anyone who can read it. This is often useful for notes from one player to another. You create the letter as a child of the generic letter, $letter (see 'help @create' and 'help write'), encrypt it so that only you and the other player can read it (see 'help encrypt') and then either give it to the player in question or leave it where they will find it. Once they've read it, they can use the 'burn' command to recycle the letter; see 'help burn' for details.