I'm not a grammar cop, but I do like to help people. I enjoy Facebook, and I read a lot of posts from intelligent people who seem to have trouble with a few words. They're not the only ones who make little mistakes with words, and I'd like to take a shot at making things easier to understand than grammar books do.
Your -- means ownership of some sort. Your Momma was a typical, grammatically correct insult when I was a kid.
You're -- means You are. I get a chuckle when I see someone on Facebook post, Your a moron. That phrasing, that misused word makes the insult ricochet.
Ur-- Ur is often used as shorthand for you're. I suppose it's okay to use that if you're (you are) texting while driving in heavy traffic, while sitting next to a moron...but it's better to take the time. As it happens, Ur was a city-state in ancient times. If you're referring to your history lessons and want to speak about Ur, then you're fine to use Ur in a sentence. It's highly unlikely that's what you're doing with your Ur, but it is possible.
Now let's take a look at places versus ownership.
There -- Is a place that isn't here.
Their -- ownership.
They're -- They are
They're headed for their car over there.
Have a great day, wherever you are...but not wherever you're. (that's an entirely different can of worms, isn't it?)