YOUR is possessive:
YOUR house is lovely.
I can't stand YOUR brother.
That's YOUR opinion, not mine.
I like the color of YOUR hair.
YOU'RE is a contraction that is short for "YOU ARE":
YOU'RE a pain in the neck. (short for "You are a pain in the neck.")
Tell me why YOU'RE always late for class.(short for "Tell me why you are always late for class.")
They told me YOU'RE a wonderful singer. (short for "They told me you are a wonderful singer.")
YOU'RE the one responsible for cleaning the house. (short for "You are the one responsible for cleaning the house.)
An easy way to get it right is to ask yourself if you mean "YOU ARE." If you do, then use the contraction
form "YOU'RE."