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.