You're is the contraction for "you are."
Your shows possession, something belongs to you or is related to you (e.g., your car, your father).
You're is a contraction of you are. It has no other uses. This is an absolute rule. If you cannot expand it to you are in your sentence, then it is wrong.
The word your sits before another word (usually a noun or a pronoun) to show that it belongs to "you." Your is a possessive adjective. (Other possessive adjectives are: my, your, his, her, its, our and their.)
discussion online
Q: Your is sometimes used these days for you’re (“you are”). Is the misuse of your a result of ignorance, or is the contraction now formally dead?
A: I think its bin caused buy spelling checkers. People think their isn't any reason to proof reed there work, since spelling checkers catch awl they're miss takes. – Peter Shor
A: It's far from universal and I certainly wouldn't hire anybody that didn't know the difference. – Jim
A: Ignorance is bliss for some. It's not a contraction at all. It's not likely to catch on for the same reason that we don't change the result for a small percentage of the population believes dividing by zero gives a zero answer. – Chris
The Urban Dictionary says Your is First grade chimpanzee for "You're" [define http://www.urbandictionary.com/define.php?term=your]
If you're not going to write it properly and you're not going to get a dictionary and you're not going to bother learning how to spell and you're too lazy to care, then write it "YOU ARE"! Your is not a contraction of you are, just as there or their is not a contraction of they are.
The incorrect usage of "your" shows up at #1 on the coppyblogger "Five Grammatical Errors that Make You Look Dumb."