Your is a
pronoun, or sometimes a
possesive adjective. You're is a
contraction of you are. Allow me to
demonstrate.
Your dog is ugly.
You're an idiot
I can't believe you're going to do that to your girlfriend.
Sometimes I think you're smarter than your actions indicate, but then I realize you're not.
The easiest way to tell which to use is try to replace it with you are and see if it works.