tell tales means to talk about things that should remain secret; a false or unkind account. For example: Children shouldn't tell tales. (= Children shouldn't tell lies about each other.)
tell-tale (adj.) means that makes a fact known, especially an unpleasant fact. For example: The murderer was given away by a few tell-tale bloodstains on his car seat.
telltale (n.) means a person who informs about other people's secrets, wrong actions, etc.