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.