Your is grammatically defined as a possessive pronominal adjective; it means "belonging to," "done by you": "That is your opinion." "You must have lost your mind." Your should not be confused with the contraction you're, meaning "you are": "This is your decision and you're going to have to live with it." The form yours is used in any construction where your will not fit: "This is a letter of yours." "The complimentary close of the letter was 'Yours truly. '" "This is yours and your sister's affair." Yours is never written with an apostrophe: if there were such a term as your's, which there isn't, it would have to mean "your is," an obvious illiteracy.
Dictionary of problem words and expressions. Harry Shaw. 1975.