a letter drafted to someone in order to tell them what you appreciate about them, what their flaws are, and how you like them anyway.
it differs from a love letter in that it doesn't have to be sweet or romantic. a like letter can be just as easily given between friends as SOs. the important thing is to be genuine and honest and not let your emotions cloud things the way they might with a love letter.
note: reactions may vary.
i gave a like letter to valerie the other day and she hugged/slapped/kissed/laughed at me. i think it went pretty well.