This is one of the most common confusions. The problem here is similar to that of there and they're, in that you're is also a contraction.
You're is really you are, the apostrophe showing the a is missing:
e.g. "You're really tall and your spiky hair makes you seem taller!"
Your is a possessive pronoun (used with hair above)
e.g. your car, your phone, etc.
One common misuse of the word your is in questions such as:
"Hope your okay."
It should, of course, be
"Hope you're okay."